feat: remove old vanilla JS/CSS frontend, make React SPA the sole entry for ccw view

Remove the entire old template-based frontend (~106K lines) and make the React
SPA the only way to access the ccw dashboard via `ccw view`.

Key changes:
- Delete all old frontend files: dashboard-css/ (37 CSS), dashboard-js/ (59 JS),
  assets/, dashboard.html, and legacy HTML templates
- Delete dashboard-generator.ts and dashboard-generator-patch.ts
- Simplify server.ts: remove ~234 lines of old frontend code (template constants,
  MODULE_CSS_FILES/MODULE_FILES arrays, generateServerDashboard(), /assets/* serving)
- Rebase React frontend from /react/ to root / (vite.config.ts, react-frontend.ts)
- Add /react/* -> /* 301 redirect for backward compatibility
- Remove --frontend and --new CLI flags from view and serve commands
- Remove generateDashboard export from public API (index.ts)
- Simplify serve.ts and view.ts to always use React without conditional branching
- Update all affected tests (unit, e2e) for React-only architecture

BREAKING CHANGE: --frontend and --new CLI flags removed; generateDashboard export
removed from ccw package; /react/ base path changed to /

ccw/src/index.ts

@@ -6,5 +6,4 @@
 export { run } from './cli.js';
 export { scanSessions } from './core/session-scanner.js';
 export { aggregateData } from './core/data-aggregator.js';
-export { generateDashboard } from './core/dashboard-generator.js';
 export { CacheManager, createDashboardCache } from './core/cache-manager.js';