feat: React WebUI replacing embedded HTML templates

[AprShine]

Summary

• Replace the embedded Jinja HTML templates with a modern React webui (React 19 + TypeScript + Vite + Tailwind CSS)
• Public page: quick/custom send modes, source selection, real-time WebSocket progress, result panel
• Admin page: dashboard with run controls, config management (LLM/Email/Sources/Schedule), history records
• Add backend connectivity banner that warns when the server is unreachable
• Add PyWebView + PyInstaller desktop packaging support (desktop.py)

Backend Changes (web_server.py)

All changes are backward-compatible — when webui/dist does not exist, the old Jinja templates are served as before:

• Register MIME types for .js/.css/.svg to fix Windows text/plain issue
• Auto-serve webui/dist build artifacts when available (/, /admin, /assets/*, /icons/*)
• Add SPA fallback route for client-side routing
• Add /health endpoint

Test plan

• python web_server.py still works with old templates (no webui/dist)
• cd webui && npm run build && cd .. && python web_server.py serves React UI
• python desktop.py launches native desktop window via PyWebView
• PyInstaller packaging produces working iDeer.exe