feat: Add AI-generated commit messages

Integrate Claude Haiku to automatically generate commit messages when
committing worktree changes. Shows a sparkle animation while generating
and auto-populates the commit message field.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

apps/ui/src/lib/electron.ts

@@ -1537,6 +1537,15 @@ function createMockWorktreeAPI(): WorktreeAPI {
       };
     },
 
+    generateCommitMessage: async (worktreePath: string) => {
+      console.log('[Mock] Generating commit message:', { worktreePath });
+      return {
+        success: true,
+        message:
+          'feat: Add new feature implementation\n\nThis is a mock AI-generated commit message.',
+      };
+    },
+
     push: async (worktreePath: string, force?: boolean) => {
       console.log('[Mock] Pushing worktree:', { worktreePath, force });
       return {