feat(updater): show in-app progress during app update

Replace native update dialogs with an in-app modal that shows a live
download progress bar and installing state, so updates no longer feel
abrupt. Drives the plugin's downloadAndInstall progress events through a
zustand store.

src/styles/app.css

@@ -1615,6 +1615,72 @@ body.resizing .sidebar {
   color: var(--ink);
 }
 
+.update-notes {
+  max-height: 180px;
+  overflow-y: auto;
+  padding: 12px 14px;
+  border: 1px solid var(--line);
+  border-radius: var(--r-sm);
+  background: var(--raised);
+  font-size: var(--t-3);
+  line-height: 1.55;
+  color: var(--ink-soft);
+  white-space: pre-line;
+}
+
+.update-status-row {
+  display: flex;
+  align-items: center;
+  gap: 11px;
+  font-size: var(--t-4);
+  color: var(--ink-soft);
+}
+
+.update-spinner {
+  flex: none;
+  width: 18px;
+  height: 18px;
+  border: 2px solid var(--line-strong);
+  border-top-color: var(--accent);
+  border-radius: 50%;
+  animation: spin 0.7s linear infinite;
+}
+
+.update-progress {
+  width: 100%;
+  height: 7px;
+  border-radius: 999px;
+  background: var(--accent-wash);
+  overflow: hidden;
+}
+
+.update-progress-fill {
+  height: 100%;
+  border-radius: 999px;
+  background: var(--accent);
+  transition: width 160ms var(--ease);
+}
+
+.update-progress.indeterminate .update-progress-fill {
+  width: 40%;
+  animation: update-indeterminate 1.1s ease-in-out infinite;
+}
+
+@keyframes update-indeterminate {
+  0% {
+    transform: translateX(-120%);
+  }
+  100% {
+    transform: translateX(320%);
+  }
+}
+
+.update-status {
+  margin: 0;
+  font-size: var(--t-3);
+  color: var(--ink-soft);
+}
+
 .btn-primary,
 .btn-ghost,
 .btn-danger {