feat(ui): branch selectors in the GitLab merge request create form

packages/web/server/lib/gitlab/DOCUMENTATION.md

@@ -101,7 +101,7 @@ Nothing in the client or repo layers assumes the token came from a PAT.
 | POST | `/api/gitlab/mrs/create` | body `{ directory, title, sourceBranch, targetBranch, description?, removeSourceBranch? }` -> `{ connected, repo?, mr }`; `400` for missing fields, unresolvable repo, or a token without the `api` scope |
 | PUT | `/api/gitlab/mrs/update` | body `{ directory, number, title?, description? }` -> `{ connected, repo?, mr }`; `404` when the MR does not exist |
 | PUT | `/api/gitlab/mrs/merge` | body `{ directory, number, squash? }` -> `{ connected, merged: true }` on success; non-mergeable MRs -> the GitLab status (`405`/`406`/`409`/`422`) with `{ connected, merged: false, message }` |
-| GET | `/api/gitlab/repo/branches` | `?namespace&project` -> `{ branches[] }` |
+| GET | `/api/gitlab/repo/branches` | `?namespace&project` -> `{ branches[], defaultBranch? }` (`defaultBranch` is `null` when the repo has no marked default branch or GitLab is disconnected) |
 
 Conventions mirror `github/routes.js`:
 

packages/web/server/lib/gitlab/routes.js

@@ -876,10 +876,11 @@ export function registerGitLabRoutes(app, options = {}) {
 
       const client = await getClient();
       if (!client) {
-        return res.json({ branches: [] });
+        return res.json({ branches: [], defaultBranch: null });
       }
 
       const branches = [];
+      let defaultBranch = null;
       let page = 1;
       while (page <= 10) {
         const resp = await client.branches(`${namespace}/${project}`, { per_page: 100, page });
@@ -893,6 +894,9 @@ export function registerGitLabRoutes(app, options = {}) {
         for (const branch of chunk) {
           if (typeof branch?.name === 'string') {
             branches.push(branch.name);
+            if (defaultBranch === null && branch.default === true) {
+              defaultBranch = branch.name;
+            }
           }
         }
         if (chunk.length < 100 || !resp.page?.hasMore) {
@@ -901,7 +905,7 @@ export function registerGitLabRoutes(app, options = {}) {
         page += 1;
       }
 
-      return res.json({ branches });
+      return res.json({ branches, defaultBranch });
     } catch (error) {
       console.error('Failed to fetch GitLab repo branches:', error);
       return res.status(500).json({ error: error.message || 'Failed to fetch GitLab repo branches' });

packages/web/server/lib/gitlab/routes.test.js

@@ -490,7 +490,20 @@ describe('GitLab data routes', () => {
     expect(response.body.diff).toContain('line two');
   });
 
-  test('repo/branches returns branch names', async () => {
+  test('repo/branches returns branch names and the default branch', async () => {
+    scriptedFetch([
+      (url) => (matches(/\/repository\/branches\?/)(url)
+        ? jsonResponse([{ name: 'main', default: true }, { name: 'feat/api' }])
+        : null),
+    ]);
+
+    const app = createApp();
+    const response = await request(app).get('/api/gitlab/repo/branches?namespace=group&project=sub');
+    expect(response.status).toBe(200);
+    expect(response.body).toEqual({ branches: ['main', 'feat/api'], defaultBranch: 'main' });
+  });
+
+  test('repo/branches returns null defaultBranch when no branch is marked default', async () => {
     scriptedFetch([
       (url) => (matches(/\/repository\/branches\?/)(url)
         ? jsonResponse([{ name: 'main' }, { name: 'feat/api' }])
@@ -500,7 +513,15 @@ describe('GitLab data routes', () => {
     const app = createApp();
     const response = await request(app).get('/api/gitlab/repo/branches?namespace=group&project=sub');
     expect(response.status).toBe(200);
-    expect(response.body).toEqual({ branches: ['main', 'feat/api'] });
+    expect(response.body).toEqual({ branches: ['main', 'feat/api'], defaultBranch: null });
+  });
+
+  test('repo/branches returns empty branches and null defaultBranch when not connected', async () => {
+    clearGitLabAuth();
+    const app = createApp();
+    const response = await request(app).get('/api/gitlab/repo/branches?namespace=group&project=sub');
+    expect(response.status).toBe(200);
+    expect(response.body).toEqual({ branches: [], defaultBranch: null });
   });
 
   test('repo/branches requires namespace and project', async () => {