feat: add MergeDialog component and integrate with GitToolbar

- Create MergeDialog component for branch merging
- Add merge button to GitToolbar
- Show source branch selector and target branch display
- Handle merge success and error states
- Add modal CSS styles

Completes Task 6.2 of git-control change.

apps/web/src/components/git-toolbar.tsx

@@ -9,6 +9,7 @@ import {
   pushRepository,
   type GitStatus,
 } from "../api/git_repositories";
+import { MergeDialog } from "./merge-dialog";
 
 interface GitToolbarProps {
   projectId: string;
@@ -33,6 +34,7 @@ export const GitToolbar = ({
   const [showNewBranch, setShowNewBranch] = useState(false);
   const [newBranchName, setNewBranchName] = useState("");
   const [newBranchBase, setNewBranchBase] = useState("");
+  const [showMergeDialog, setShowMergeDialog] = useState(false);
 
   const loadStatus = useCallback(async () => {
     try {
@@ -179,6 +181,14 @@ export const GitToolbar = ({
             ⇡ Push
             {status?.ahead ? <span className="badge">{status.ahead}</span> : null}
           </button>
+          <button
+            className="toolbar-button"
+            onClick={() => setShowMergeDialog(true)}
+            disabled={loading}
+            type="button"
+          >
+            🔀 Merge
+          </button>
         </div>
       </div>
 
@@ -227,6 +237,19 @@ export const GitToolbar = ({
           {status.untracked.length > 0 && <span className="status-badge untracked">❓ {status.untracked.length} untracked</span>}
         </div>
       )}
+
+      <MergeDialog
+        projectId={projectId}
+        repoId={repoId}
+        branches={branches}
+        currentBranch={currentBranch}
+        isOpen={showMergeDialog}
+        onClose={() => setShowMergeDialog(false)}
+        onMerge={() => {
+          void loadStatus();
+          onRefresh();
+        }}
+      />
     </div>
   );
 };