feat(21-04): create ChatPanelContext, ChatInput, and ChatMessage components

- ChatPanelContext with localStorage persistence (nexus:chat-panel-open)
- ChatInput with Enter/Shift+Enter/Escape keyboard shortcuts and auto-resize
- ChatMessage renders user bubbles (bg-secondary) and assistant markdown via ChatMarkdownMessage
- ChatInput.test.tsx with 6 passing tests (keyboard shortcuts, max-height, submit state)

ui/src/components/ChatInput.test.tsx

@@ -1,19 +1,166 @@
-// @vitest-environment node
-import { describe, it } from "vitest";
+// @vitest-environment jsdom
+
+import { act } from "react";
+import { createRoot } from "react-dom/client";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { ChatInput } from "./ChatInput";
+
+// Tell React this environment uses act() for event flushing.
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+(globalThis as any).IS_REACT_ACT_ENVIRONMENT = true;
 
 describe("ChatInput", () => {
+  let container: HTMLDivElement;
+  let root: ReturnType<typeof createRoot> | null = null;
+
+  beforeEach(() => {
+    container = document.createElement("div");
+    document.body.appendChild(container);
+    root = null;
+  });
+
+  afterEach(() => {
+    if (root) {
+      act(() => {
+        root!.unmount();
+      });
+      root = null;
+    }
+    if (container.parentNode) {
+      container.remove();
+    }
+  });
+
+  function renderChatInput(props: { onSend: (content: string) => void; isSubmitting?: boolean; disabled?: boolean }) {
+    root = createRoot(container);
+    act(() => {
+      root!.render(<ChatInput {...props} />);
+    });
+    return {
+      getTextarea: () => container.querySelector("textarea")!,
+      getSendButton: () => container.querySelector("button[aria-label='Send message']") as HTMLButtonElement | null,
+    };
+  }
+
   describe("keyboard shortcuts (INPUT-07)", () => {
-    it.todo("calls onSend when Enter is pressed without Shift");
-    it.todo("inserts newline when Shift+Enter is pressed");
-    it.todo("clears input when Escape is pressed");
-    it.todo("does not send when input is empty");
+    it("calls onSend when Enter is pressed without Shift", () => {
+      const onSend = vi.fn();
+      const { getTextarea } = renderChatInput({ onSend });
+      const textarea = getTextarea();
+
+      act(() => {
+        // Set value
+        Object.defineProperty(textarea, "value", {
+          writable: true,
+          value: "Hello world",
+        });
+        const inputEvent = new Event("input", { bubbles: true });
+        textarea.dispatchEvent(inputEvent);
+      });
+
+      act(() => {
+        const keydown = new KeyboardEvent("keydown", {
+          key: "Enter",
+          bubbles: true,
+          cancelable: true,
+          shiftKey: false,
+        });
+        textarea.dispatchEvent(keydown);
+      });
+
+      expect(onSend).toHaveBeenCalledWith("Hello world");
+    });
+
+    it("inserts newline when Shift+Enter is pressed", () => {
+      const onSend = vi.fn();
+      const { getTextarea } = renderChatInput({ onSend });
+      const textarea = getTextarea();
+
+      act(() => {
+        Object.defineProperty(textarea, "value", {
+          writable: true,
+          value: "Hello",
+        });
+        const inputEvent = new Event("input", { bubbles: true });
+        textarea.dispatchEvent(inputEvent);
+      });
+
+      act(() => {
+        const keydown = new KeyboardEvent("keydown", {
+          key: "Enter",
+          bubbles: true,
+          cancelable: true,
+          shiftKey: true,
+        });
+        textarea.dispatchEvent(keydown);
+      });
+
+      // onSend should NOT have been called
+      expect(onSend).not.toHaveBeenCalled();
+    });
+
+    it("clears input when Escape is pressed", () => {
+      const onSend = vi.fn();
+      const { getTextarea } = renderChatInput({ onSend });
+      const textarea = getTextarea();
+
+      act(() => {
+        Object.defineProperty(textarea, "value", {
+          writable: true,
+          value: "Some text",
+        });
+        const inputEvent = new Event("input", { bubbles: true });
+        textarea.dispatchEvent(inputEvent);
+      });
+
+      act(() => {
+        const keydown = new KeyboardEvent("keydown", {
+          key: "Escape",
+          bubbles: true,
+          cancelable: true,
+        });
+        textarea.dispatchEvent(keydown);
+      });
+
+      expect(textarea.value).toBe("");
+    });
+
+    it("does not send when input is empty", () => {
+      const onSend = vi.fn();
+      const { getTextarea } = renderChatInput({ onSend });
+      const textarea = getTextarea();
+
+      act(() => {
+        const keydown = new KeyboardEvent("keydown", {
+          key: "Enter",
+          bubbles: true,
+          cancelable: true,
+          shiftKey: false,
+        });
+        textarea.dispatchEvent(keydown);
+      });
+
+      expect(onSend).not.toHaveBeenCalled();
+    });
   });
 
   describe("auto-resize (INPUT-01)", () => {
-    it.todo("textarea has max-height constraint");
+    it("textarea has max-height constraint", () => {
+      const onSend = vi.fn();
+      const { getTextarea } = renderChatInput({ onSend });
+      const textarea = getTextarea();
+      // The max-h-[160px] class should be on the textarea
+      expect(textarea.className).toContain("max-h-[160px]");
+    });
   });
 
   describe("submit state", () => {
-    it.todo("disables send button when isSubmitting is true");
+    it("disables send button when isSubmitting is true", () => {
+      const onSend = vi.fn();
+      const { getSendButton } = renderChatInput({ onSend, isSubmitting: true });
+      const btn = getSendButton();
+      expect(btn).not.toBeNull();
+      expect(btn!.disabled).toBe(true);
+    });
   });
 });

ui/src/components/ChatInput.tsx

@@ -0,0 +1,92 @@
+import { useEffect, useRef, useState } from "react";
+import { Send, Loader2 } from "lucide-react";
+import { Button } from "@/components/ui/button";
+import { cn } from "../lib/utils";
+
+interface ChatInputProps {
+  onSend: (content: string) => void;
+  isSubmitting?: boolean;
+  disabled?: boolean;
+}
+
+export function ChatInput({ onSend, isSubmitting = false, disabled = false }: ChatInputProps) {
+  const [value, setValue] = useState("");
+  const textareaRef = useRef<HTMLTextAreaElement>(null);
+
+  // Auto-resize fallback for browsers without field-sizing: content support
+  useEffect(() => {
+    const el = textareaRef.current;
+    if (!el) return;
+    el.style.height = "auto";
+    el.style.height = `${el.scrollHeight}px`;
+  }, [value]);
+
+  function submit() {
+    const trimmed = value.trim();
+    if (!trimmed || isSubmitting || disabled) return;
+    onSend(trimmed);
+    setValue("");
+    if (textareaRef.current) {
+      textareaRef.current.style.height = "auto";
+    }
+  }
+
+  function handleKeyDown(e: React.KeyboardEvent<HTMLTextAreaElement>) {
+    if (e.key === "Enter" && !e.shiftKey && !e.nativeEvent.isComposing) {
+      e.preventDefault();
+      submit();
+    } else if (e.key === "Escape") {
+      e.preventDefault();
+      setValue("");
+      if (textareaRef.current) {
+        textareaRef.current.style.height = "auto";
+      }
+    }
+    // Shift+Enter falls through to default behavior (inserts newline)
+  }
+
+  const isEmpty = value.trim().length === 0;
+
+  return (
+    <form
+      onSubmit={(e) => {
+        e.preventDefault();
+        submit();
+      }}
+      className="flex items-end gap-2"
+    >
+      <textarea
+        ref={textareaRef}
+        value={value}
+        onChange={(e) => setValue(e.target.value)}
+        onKeyDown={handleKeyDown}
+        placeholder="Message your agent..."
+        rows={1}
+        disabled={disabled}
+        className={cn(
+          "flex w-full rounded-md border border-input bg-transparent px-3 py-2 text-sm",
+          "placeholder:text-muted-foreground",
+          "focus-visible:outline-none focus-visible:ring-1 focus-visible:ring-ring",
+          "disabled:cursor-not-allowed disabled:opacity-50",
+          "resize-none min-h-[40px] max-h-[160px] overflow-y-auto",
+          // field-sizing: content for modern browsers (Chrome 123+, Firefox 129+)
+          "[field-sizing:content]",
+        )}
+      />
+      <Button
+        type="submit"
+        variant="ghost"
+        size="icon"
+        disabled={isEmpty || isSubmitting || disabled}
+        aria-label="Send message"
+        className="shrink-0"
+      >
+        {isSubmitting ? (
+          <Loader2 className="h-4 w-4 animate-spin" />
+        ) : (
+          <Send className="h-4 w-4" />
+        )}
+      </Button>
+    </form>
+  );
+}

ui/src/components/ChatMessage.tsx

@@ -0,0 +1,29 @@
+import { ChatMarkdownMessage } from "./ChatMarkdownMessage";
+import { cn } from "../lib/utils";
+
+interface ChatMessageProps {
+  role: "user" | "assistant" | "system";
+  content: string;
+}
+
+export function ChatMessage({ role, content }: ChatMessageProps) {
+  if (role === "user") {
+    return (
+      <div className="flex justify-end">
+        <div
+          className={cn(
+            "max-w-[85%] rounded-lg bg-secondary px-3 py-2 text-secondary-foreground text-sm",
+          )}
+        >
+          {content}
+        </div>
+      </div>
+    );
+  }
+  // assistant or system
+  return (
+    <div className="max-w-full">
+      <ChatMarkdownMessage content={content} />
+    </div>
+  );
+}

ui/src/context/ChatPanelContext.tsx

@@ -0,0 +1,62 @@
+import { createContext, useCallback, useContext, useState, type ReactNode } from "react";
+
+const STORAGE_KEY = "nexus:chat-panel-open";
+
+interface ChatPanelContextValue {
+  chatOpen: boolean;
+  activeConversationId: string | null;
+  setChatOpen: (open: boolean) => void;
+  toggleChat: () => void;
+  setActiveConversationId: (id: string | null) => void;
+}
+
+const ChatPanelContext = createContext<ChatPanelContextValue | null>(null);
+
+function readPreference(): boolean {
+  try {
+    const raw = localStorage.getItem(STORAGE_KEY);
+    return raw === "true";
+  } catch {
+    return false;
+  }
+}
+
+function writePreference(open: boolean) {
+  try {
+    localStorage.setItem(STORAGE_KEY, String(open));
+  } catch {
+    /* ignore */
+  }
+}
+
+export function ChatPanelProvider({ children }: { children: ReactNode }) {
+  const [chatOpen, setChatOpenState] = useState(readPreference);
+  const [activeConversationId, setActiveConversationId] = useState<string | null>(null);
+
+  const setChatOpen = useCallback((open: boolean) => {
+    setChatOpenState(open);
+    writePreference(open);
+  }, []);
+
+  const toggleChat = useCallback(() => {
+    setChatOpenState((prev) => {
+      const next = !prev;
+      writePreference(next);
+      return next;
+    });
+  }, []);
+
+  return (
+    <ChatPanelContext.Provider
+      value={{ chatOpen, activeConversationId, setChatOpen, toggleChat, setActiveConversationId }}
+    >
+      {children}
+    </ChatPanelContext.Provider>
+  );
+}
+
+export function useChatPanel() {
+  const ctx = useContext(ChatPanelContext);
+  if (!ctx) throw new Error("useChatPanel must be used within ChatPanelProvider");
+  return ctx;
+}