Branch menu UI and remote branch checkout

Add UI improvements and switching state for branch menu and implement remote-branch-aware checkout logic.

Frontend: add CSS for disabled branch items, compact branch name layout, subtitles and messages; show switching indicator and disable branch actions while switching; display local/remote names and remote subtitles; surface branch switch errors and in-progress messages. Add state.branches.switchingTo and update event handlers to set/clear it.

Backend (tauri): extend LocalRepoBranch with is_remote, remote_name, local_name; add git_ref_exists helper; enhance checkout_branch to detect local vs remote refs and automatically checkout or create tracking branches for remotes; parse refs to include remote branches (skip ones with existing local counterparts) and sort branches with current/local/remote order. Minor formatting tweaks.

frontend/css/components.css

@@ -674,6 +674,11 @@
   font-size: 13px;
 }
 
+.branch-menu-item:disabled {
+  cursor: default;
+  opacity: 0.72;
+}
+
 .branch-menu-item.active {
   background: var(--accent-subtle);
   color: var(--accent);
@@ -685,11 +690,35 @@
   border-color: transparent;
 }
 
+.branch-menu-item:disabled:hover {
+  background: transparent;
+}
+
+.branch-menu-name {
+  min-width: 0;
+  display: flex;
+  flex-direction: column;
+  gap: 2px;
+}
+
+.branch-menu-name > span {
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
+.branch-menu-subtitle,
 .branch-current-mark {
   color: var(--text-muted);
   font-size: 11px;
 }
 
+.branch-menu-message {
+  padding: 6px 8px;
+  color: var(--text-muted);
+  font-size: 12px;
+}
+
 .danger-subtle {
   color: var(--danger);
 }