- Create ChatFileDropZone component with drag-and-drop state and overlay
- Add onFilesPicked/pendingFiles/onRemoveFile props to ChatInput
- Wrap form in ChatFileDropZone for drag-and-drop support
- Add handlePaste for clipboard image paste (clipboardData.files)
- Add Paperclip icon button with hidden file input for file picker
- Show pending file chips above textarea with progress and remove button
- Add tests: renders file attach button, calls onFilesPicked, shows pending chips
