fix: merge worktree handler now uses correct branch name and path

The merge handler previously hardcoded branch names as `feature/${featureId}`
and worktree paths as `.worktrees/${featureId}`, which failed for auto-generated
branches (e.g., `feature/v0.11.0rc-1768413895104-31pa`) and custom worktrees.

Changes:
- Server handler now accepts branchName and worktreePath directly from the UI
- Added branch existence validation before attempting merge
- Updated merge dialog with 2-step confirmation (type "merge" to confirm)
- Removed feature branch naming restriction - any branch can now be merged
- Updated API types and client to pass correct parameters

Closes #408

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

apps/ui/src/lib/electron.ts

@@ -1440,13 +1440,19 @@ function createMockSetupAPI(): SetupAPI {
 // Mock Worktree API implementation
 function createMockWorktreeAPI(): WorktreeAPI {
   return {
-    mergeFeature: async (projectPath: string, featureId: string, options?: object) => {
+    mergeFeature: async (
+      projectPath: string,
+      branchName: string,
+      worktreePath: string,
+      options?: object
+    ) => {
       console.log('[Mock] Merging feature:', {
         projectPath,
-        featureId,
+        branchName,
+        worktreePath,
         options,
       });
-      return { success: true, mergedBranch: `feature/${featureId}` };
+      return { success: true, mergedBranch: branchName };
     },
 
     getInfo: async (projectPath: string, featureId: string) => {