refactor: extract global styles and tokens (Task 2.1)

- Create styles/tokens.css with CSS custom properties and dark theme
- Create styles/global.css with resets, shell layout, and navigation
- Create styles/utilities.css with generic utilities and primitives
- Create styles/syntax-highlight.css with Prism.js theme
- Update main.tsx to import the 4 new style files
- Keep styles.css intact for backward compatibility

Quality gates: build (pass), lint (pass)
Refs: repo-restructure Task 2.1

apps/web/src/main.tsx

@@ -5,7 +5,10 @@ import { BrowserRouter } from "react-router-dom";
 import { AppRouter } from "./router";
 import { AuthProvider } from "./state/auth";
 import { SessionsProvider } from "./state/sessions";
-import "./styles.css";
+import "./styles/tokens.css";
+import "./styles/global.css";
+import "./styles/utilities.css";
+import "./styles/syntax-highlight.css";
 
 ReactDOM.createRoot(document.getElementById("root")!).render(
   <React.StrictMode>