feat: add in-app update notification banner to sidebar

Subscribe to auto-update status at the App level and render a compact
UpdateBanner in the sidebar footer when an update is available,
downloading, or downloaded:

- Available/downloading: subtle banner with progress bar, dismissable
- Downloaded: prominent 'Restart to update' action with glow effect

Clicking the banner opens Settings directly on the Troubleshooting
section via a new initialSection prop on SettingsPanel.

New files:
- src/renderer/components/ui/UpdateBanner.tsx

Modified files:
- App.tsx: subscribe to onUpdateStatus, wire props
- Sidebar.tsx: accept and render UpdateBanner
- SettingsPanel.tsx: add initialSection prop + export SettingsSection type
- styles.css: add update-banner-enter slide-up animation

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/renderer/styles.css

@@ -626,6 +626,19 @@ body {
   animation: banner-slide-in 0.25s cubic-bezier(0.16, 1, 0.3, 1);
 }
 
+/* ── Update banner slide-up ──────────────────────────────────── */
+
+@keyframes update-banner-in {
+  from {
+    opacity: 0;
+    transform: translateY(100%);
+  }
+}
+
+.update-banner-enter {
+  animation: update-banner-in 0.3s cubic-bezier(0.16, 1, 0.3, 1);
+}
+
 /* ── Thinking process section ────────────────────────────────── */
 
 @keyframes thinking-process-in {
@@ -651,6 +664,7 @@ body {
   .msg-actions-enter,
   .session-item-enter,
   .banner-slide-enter,
+  .update-banner-enter,
   .thinking-process-enter {
     animation: none;
   }