Add collapsible sidebar with icon rail

Persist collapsed state in localStorage. When collapsed, render a
narrow rail with Projects/MCP/Settings icon buttons that expand the
sidebar to that view on click.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/src/components/layout/Sidebar.test.tsx

@@ -8,6 +8,9 @@ vi.mock("../../store/appState", () => ({
     selector({
       sidebarView: "projects",
       setSidebarView: vi.fn(),
+      sidebarCollapsed: false,
+      setSidebarCollapsed: vi.fn(),
+      toggleSidebarCollapsed: vi.fn(),
     })
   ),
 }));