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/App.tsx

@@ -36,6 +36,9 @@ import { Badge } from '@/components/ui/badge'
 const STORAGE_KEY = 'autocoder-selected-project'
 const VIEW_MODE_KEY = 'autocoder-view-mode'
 
+// Bottom padding for main content when debug panel is collapsed (40px header + 8px margin)
+const COLLAPSED_DEBUG_PANEL_CLEARANCE = 48
+
 function App() {
   // Initialize selected project from localStorage
   const [selectedProject, setSelectedProject] = useState<string | null>(() => {
@@ -331,7 +334,7 @@ function App() {
       {/* Main Content */}
       <main
         className="max-w-7xl mx-auto px-4 py-8"
-        style={{ paddingBottom: debugOpen ? debugPanelHeight + 32 : 48 }}
+        style={{ paddingBottom: debugOpen ? debugPanelHeight + 32 : COLLAPSED_DEBUG_PANEL_CLEARANCE }}
       >
         {!selectedProject ? (
           <div className="text-center mt-12">