refactor(nexus): apply Task 6 code-quality fixes to Layout.tsx (phase 8)

Three cleanups from the Task 6 code-quality review:

1. Delete dead instanceSettingsTarget state and supporting code.
   The useState, the readRememberedInstanceSettingsPath helper, the
   INSTANCE_SETTINGS_MEMORY_KEY constant, and the localStorage-writing
   effect were all residue from the old footer sidebar buttons that
   Task 6 deleted. Verified via grep that no other file in ui/src
   reads paperclip.lastInstanceSettingsPath.

2. Consolidate two near-identical mobile-nav-visibility effects into
   one. The standalone isMobile-reset effect was a strict subset of
   the scroll-listener effect's early-return branch.

3. Drop onToggleSidebar / onTogglePanel stub callbacks.
   useKeyboardShortcuts declares these as optional and uses optional
   chaining internally; the stubs were "type contract satisfaction"
   the contract did not actually require.

No behavioral change. 38 frame tests still pass.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/components/Layout.tsx

@@ -23,25 +23,10 @@ import { useCompanyPageMemory } from "../hooks/useCompanyPageMemory";
 import { healthApi } from "../api/health";
 import { instanceSettingsApi } from "../api/instanceSettings";
 import { shouldSyncCompanySelectionFromRoute } from "../lib/company-selection";
-import {
-  DEFAULT_INSTANCE_SETTINGS_PATH,
-  normalizeRememberedInstanceSettingsPath,
-} from "../lib/instance-settings";
 import { queryKeys } from "../lib/queryKeys";
 import { cn } from "../lib/utils";
 import { NotFoundPage } from "../pages/NotFound";
 
-const INSTANCE_SETTINGS_MEMORY_KEY = "paperclip.lastInstanceSettingsPath";
-
-function readRememberedInstanceSettingsPath(): string {
-  if (typeof window === "undefined") return DEFAULT_INSTANCE_SETTINGS_PATH;
-  try {
-    return normalizeRememberedInstanceSettingsPath(window.localStorage.getItem(INSTANCE_SETTINGS_MEMORY_KEY));
-  } catch {
-    return DEFAULT_INSTANCE_SETTINGS_PATH;
-  }
-}
-
 export function Layout() {
   const { isMobile } = useSidebar();
   const { openNewIssue, openOnboarding } = useDialog();
@@ -59,7 +44,6 @@ export function Layout() {
   const onboardingTriggered = useRef(false);
   const lastMainScrollTop = useRef(0);
   const [mobileNavVisible, setMobileNavVisible] = useState(true);
-  const [, setInstanceSettingsTarget] = useState<string>(() => readRememberedInstanceSettingsPath());
 
   const matchedCompany = useMemo(() => {
     if (!companyPrefix) return null;
@@ -144,13 +128,6 @@ export function Layout() {
   useKeyboardShortcuts({
     enabled: keyboardShortcutsEnabled,
     onNewIssue: () => openNewIssue(),
-    onToggleSidebar: () => {
-      // Phase 8: sidebar toggle is a no-op from keyboard — the rail is fixed.
-      // Kept as a stub so useKeyboardShortcuts' type contract is satisfied.
-    },
-    onTogglePanel: () => {
-      // Phase 8: PropertiesPanel is no longer mounted globally.
-    },
     onSearch: () => {
       // Phase 8: open the command palette via synthetic keydown, mirroring
       // the CmdKButton shim. Phase 14 replaces with a real palette context.
@@ -158,15 +135,6 @@ export function Layout() {
     },
   });
 
-  useEffect(() => {
-    if (!isMobile) {
-      setMobileNavVisible(true);
-      return;
-    }
-    lastMainScrollTop.current = 0;
-    setMobileNavVisible(true);
-  }, [isMobile]);
-
   const updateMobileNavVisibility = useCallback((currentTop: number) => {
     const delta = currentTop - lastMainScrollTop.current;
     if (currentTop <= 24) setMobileNavVisible(true);
@@ -176,11 +144,9 @@ export function Layout() {
   }, []);
 
   useEffect(() => {
-    if (!isMobile) {
+    lastMainScrollTop.current = 0;
-      setMobileNavVisible(true);
+    setMobileNavVisible(true);
-      lastMainScrollTop.current = 0;
+    if (!isMobile) return;
-      return;
-    }
     const onScroll = () => {
       updateMobileNavVisibility(window.scrollY || document.documentElement.scrollTop || 0);
     };
@@ -197,19 +163,6 @@ export function Layout() {
     };
   }, [isMobile]);
 
-  useEffect(() => {
-    if (!location.pathname.startsWith("/instance/settings/")) return;
-    const nextPath = normalizeRememberedInstanceSettingsPath(
-      `${location.pathname}${location.search}${location.hash}`,
-    );
-    setInstanceSettingsTarget(nextPath);
-    try {
-      window.localStorage.setItem(INSTANCE_SETTINGS_MEMORY_KEY, nextPath);
-    } catch {
-      // Ignore storage failures in restricted environments.
-    }
-  }, [location.hash, location.pathname, location.search]);
-
   return (
     <GeneralSettingsProvider value={{ keyboardShortcutsEnabled }}>
       <div