feat: blend title bar, remove menu, add agent activity indicators

- Configure hidden title bar with titleBarOverlay matching dark theme
- Remove native menu bar via Menu.setApplicationMenu(null)
- Fix backgroundColor mismatch (#0f172a -> #09090b)
- Add full-width drag region in AppShell for window dragging
- Adjust sidebar and chat headers with top padding for overlay zone
- Add ThinkingDots component with animated dot sequence
- Show activity indicator in chat when agent is processing
- Replace 'Generating...' spinner with inline blinking cursor
- Refine header status badge to subtle pulsing dot
- Add BACKEND_UI_CHANGES.md documenting future sidecar protocol additions

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

src/renderer/styles.css

@@ -61,6 +61,41 @@ textarea {
   background: #52525b;
 }
 
+/* Window drag regions for custom title bar */
+.drag-region {
+  -webkit-app-region: drag;
+}
+
+.no-drag {
+  -webkit-app-region: no-drag;
+}
+
+/* Thinking dots animation */
+@keyframes thinking-dot {
+  0%,
+  60%,
+  100% {
+    opacity: 0.3;
+    transform: scale(0.8);
+  }
+  30% {
+    opacity: 1;
+    transform: scale(1);
+  }
+}
+
+.thinking-dot {
+  animation: thinking-dot 1.4s ease-in-out infinite;
+}
+
+.thinking-dot:nth-child(2) {
+  animation-delay: 0.2s;
+}
+
+.thinking-dot:nth-child(3) {
+  animation-delay: 0.4s;
+}
+
 /* Auto-resize textarea helper */
 .auto-resize-textarea {
   field-sizing: content;