fix: address second round of PR review feedback

- Add fallback for unknown enhancement modes in history button to prevent "Enhanced (undefined)" UI bug
- Move DescriptionHistoryEntry interface to top level in add-feature-dialog
- Import and use EnhancementMode type in edit-feature-dialog to eliminate hardcoded types
- Make FollowUpHistoryEntry extend BaseHistoryEntry for consistency

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

apps/ui/src/components/views/board-view/shared/enhancement/enhancement-history-button.tsx

@@ -54,7 +54,9 @@ export function EnhancementHistoryButton<T extends BaseHistoryEntry>({
       return 'Original';
     }
     if (entry.source === 'enhance') {
-      return `Enhanced (${ENHANCEMENT_MODE_LABELS[entry.enhancementMode ?? 'improve']})`;
+      const mode = entry.enhancementMode ?? 'improve';
+      const label = ENHANCEMENT_MODE_LABELS[mode as EnhancementMode] ?? mode;
+      return `Enhanced (${label})`;
     }
     return 'Edited';
   };