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/components/AppShell.tsx

@@ -9,6 +9,9 @@ interface AppShellProps {
 export function AppShell({ sidebar, content, overlay }: AppShellProps) {
   return (
     <div className="relative flex h-screen bg-[var(--color-surface-0)] text-zinc-100">
+      {/* Full-width drag region matching the title bar overlay height */}
+      <div className="drag-region absolute inset-x-0 top-0 z-10 h-10" />
+
       <aside className="flex w-72 shrink-0 flex-col border-r border-[var(--color-border)] bg-[var(--color-surface-1)]">
         {sidebar}
       </aside>