refactor(ui): extract keyboard utilities and add padding constant

- Create shared `isSubmitEnter()` utility in `ui/src/lib/keyboard.ts`
  for IME-aware Enter key handling across all input components
- Extract magic number 48 to named constant `COLLAPSED_DEBUG_PANEL_CLEARANCE`
  with explanatory comment (40px panel header + 8px margin)
- Update 5 components to use the new utility:
  - AssistantChat.tsx
  - ExpandProjectChat.tsx
  - SpecCreationChat.tsx
  - FolderBrowser.tsx
  - TerminalTabs.tsx

This follows up on PR #121 which added IME composition checks. The
refactoring centralizes the logic for easier maintenance and documents
the padding value that prevents Kanban cards from being cut off when
the debug panel is collapsed.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

ui/src/components/FolderBrowser.tsx

@@ -18,6 +18,7 @@ import {
   ArrowLeft,
 } from 'lucide-react'
 import * as api from '../lib/api'
+import { isSubmitEnter } from '../lib/keyboard'
 import type { DirectoryEntry, DriveInfo } from '../lib/types'
 import { Button } from '@/components/ui/button'
 import { Input } from '@/components/ui/input'
@@ -269,8 +270,7 @@ export function FolderBrowser({ onSelect, onCancel, initialPath }: FolderBrowser
                     className="flex-1"
                     autoFocus
                     onKeyDown={(e) => {
-                      // Skip if composing (e.g., Japanese IME input)
-                      if (e.key === 'Enter' && !e.nativeEvent.isComposing) handleCreateFolder()
+                      if (isSubmitEnter(e, false)) handleCreateFolder()
                       if (e.key === 'Escape') {
                         setIsCreatingFolder(false)
                         setNewFolderName('')