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/vite.config.ts

@@ -0,0 +1,21 @@
+import { defineConfig } from "vite";
+import react from "@vitejs/plugin-react";
+import tailwindcss from "@tailwindcss/vite";
+
+export default defineConfig({
+  plugins: [react(), tailwindcss()],
+  server: {
+    port: 5173,
+    proxy: {
+      "/api": "http://localhost:3000",
+      "/auth": "http://localhost:3000",
+      "/ws": {
+        target: "ws://localhost:3000",
+        ws: true,
+      },
+    },
+  },
+  build: {
+    outDir: "dist",
+  },
+});