feat: build the chat + preview + approve interface

- web: two-pane app chrome (chat panel + browser-framed preview)
- ChatPanel, Preview, ApproveBar components + useChat (mock agent for now)
- Fluent/Figma design system (Inter, dark chrome, elevation, motion)
- 107 unit tests + Playwright e2e green; web typechecks + builds

web/tsconfig.json

@@ -5,7 +5,8 @@
     "outDir": "dist",
     "rootDir": "src",
     "lib": ["ES2022", "DOM", "DOM.Iterable"],
-    "noEmit": true
+    "noEmit": true,
+    "allowImportingTsExtensions": true
   },
   "include": ["src"]
 }