feat(git): add tag management and cherry-pick workflow

This update extends the Git backend and UI to support listing,
creating, deleting, and pushing tags. It also adds cherry-pick
commands with proper in-progress detection and conflict handling, and
prevents other operations while a cherry-pick is active.

- Add GitTag model, tag listing, and tag CRUD/push commands
- Implement cherry-pick start/continue/abort with status tracking
- Update UI to display tags and gate actions during cherry-pick

src/app.css

@@ -1550,6 +1550,12 @@
     font-size: 10px;
   }
 
+  .tag-group-head { display: flex; align-items: center; gap: 6px; }
+  .tag-group-head .branch-group-toggle { flex: 1; min-width: 0; }
+  .tag-group-head .branch-create-toggle { flex-shrink: 0; }
+
+  .tag-create-form { grid-template-columns: auto minmax(0, 1fr) minmax(0, 1fr) auto auto; }
+
   .branch-empty {
     padding: 8px 10px;
     color: var(--color-ink-faint);