feat: add admin panel with Discord OAuth and dashboard

Adds a React admin panel (panel/) with Discord OAuth2 login,
live dashboard via WebSocket, and settings/management pages.
Includes Docker build support, Vite proxy config for dev,
game_settings migration, and open-redirect protection on auth callback.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

panel/package.json

@@ -0,0 +1,28 @@
+{
+  "name": "panel",
+  "version": "0.1.0",
+  "private": true,
+  "type": "module",
+  "scripts": {
+    "dev": "vite",
+    "build": "vite build",
+    "preview": "vite preview"
+  },
+  "dependencies": {
+    "react": "^19.1.0",
+    "react-dom": "^19.1.0",
+    "react-router-dom": "^7.6.1"
+  },
+  "devDependencies": {
+    "@types/react": "^19.1.6",
+    "@types/react-dom": "^19.1.6",
+    "@vitejs/plugin-react": "^4.6.0",
+    "autoprefixer": "^10.4.21",
+    "daisyui": "^5.0.43",
+    "postcss": "^8.5.6",
+    "tailwindcss": "^4.1.10",
+    "@tailwindcss/vite": "^4.1.10",
+    "typescript": "^5.9.3",
+    "vite": "^6.3.5"
+  }
+}