feat(#42): Implement persistent Jarvis chat overlay

Add a persistent chat overlay accessible from any authenticated view.
The overlay wraps the existing Chat component and adds state management,
keyboard shortcuts, and responsive design.

Features:
- Three states: Closed (floating button), Open (full panel), Minimized (header)
- Keyboard shortcuts:
  - Cmd/Ctrl + K: Open chat (when closed)
  - Escape: Minimize chat (when open)
  - Cmd/Ctrl + Shift + J: Toggle chat panel
- State persistence via localStorage
- Responsive design (full-width mobile, sidebar desktop)
- PDA-friendly design with calm colors
- 32 comprehensive tests (14 hook tests + 18 component tests)

Files added:
- apps/web/src/hooks/useChatOverlay.ts
- apps/web/src/hooks/useChatOverlay.test.ts
- apps/web/src/components/chat/ChatOverlay.tsx
- apps/web/src/components/chat/ChatOverlay.test.tsx

Files modified:
- apps/web/src/components/chat/index.ts (added export)
- apps/web/src/app/(authenticated)/layout.tsx (integrated overlay)

All tests passing (490 tests, 50 test files)
All lint checks passing
Build succeeds

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

apps/web/src/app/(authenticated)/layout.tsx

@@ -4,6 +4,7 @@ import { useEffect } from "react";
 import { useRouter } from "next/navigation";
 import { useAuth } from "@/lib/auth/auth-context";
 import { Navigation } from "@/components/layout/Navigation";
+import { ChatOverlay } from "@/components/chat";
 import type { ReactNode } from "react";
 
 export default function AuthenticatedLayout({
@@ -36,6 +37,7 @@ export default function AuthenticatedLayout({
     <div className="min-h-screen bg-gray-50">
       <Navigation />
       <div className="pt-16">{children}</div>
+      <ChatOverlay />
     </div>
   );
 }