feat: add desktop app with React frontend, API backend, theme system (#2)

New desktop application that launches a local HTTP server with embedded
React frontend. Opens in the user's browser automatically.

Architecture:
- internal/api/: REST API exposing all internal/ packages to frontend
- cmd/muyue-desktop/: entry point, serves embedded frontend + API
- cmd/muyue-desktop/frontend/: React + Vite SPA

Frontend features:
- 4 tabs: Dashboard, Studio, Shell, Config
- Cyberpunk red theme with CSS custom properties
- Theme system: 4 built-in themes (Cyberpunk Red, Pink, Midnight Blue, Matrix Green)
- Terminal with command execution via API
- Chat interface with sidebar (agents, workflows, commands)
- Live clock, status indicators, update badges
- Glitch/scanline/fade animations between tabs
- xterm.js included for future full terminal integration

Backend API endpoints:
- GET /api/info, /api/system, /api/tools, /api/config
- GET /api/providers, /api/skills, /api/lsp, /api/mcp, /api/updates
- POST /api/scan, /api/install, /api/terminal, /api/mcp/configure

Build: cd cmd/muyue-desktop/frontend && npm run build && go build ./cmd/muyue-desktop/
Binary: ~11MB single binary with embedded frontend

Generated with Crush

Assisted-by: GLM-5.1 via Crush <crush@charm.land>

Co-authored-by: Augustin <muyue@legion-muyue.fr>
Reviewed-on: #2

cmd/muyue-desktop/frontend/package.json

@@ -0,0 +1,26 @@
+{
+  "name": "frontend",
+  "version": "1.0.0",
+  "description": "",
+  "main": "index.js",
+  "scripts": {
+    "dev": "vite",
+    "build": "vite build",
+    "preview": "vite preview"
+  },
+  "keywords": [],
+  "author": "",
+  "license": "ISC",
+  "type": "commonjs",
+  "dependencies": {
+    "@xterm/addon-fit": "^0.11.0",
+    "@xterm/xterm": "^6.0.0",
+    "react": "^19.2.5",
+    "react-dom": "^19.2.5",
+    "xterm": "^5.3.0"
+  },
+  "devDependencies": {
+    "@vitejs/plugin-react": "^6.0.1",
+    "vite": "^8.0.9"
+  }
+}