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>

.env.example

@@ -20,6 +20,13 @@ DISCORD_BOT_TOKEN=your-discord-bot-token
 DISCORD_CLIENT_ID=your-discord-client-id
 DISCORD_GUILD_ID=your-discord-guild-id
 
+# Admin Panel (Discord OAuth)
+# Get client secret from: https://discord.com/developers/applications → OAuth2
+DISCORD_CLIENT_SECRET=your-discord-client-secret
+SESSION_SECRET=change-me-to-a-random-string
+ADMIN_USER_IDS=123456789012345678
+PANEL_BASE_URL=http://localhost:3000
+
 # Server (for remote access scripts)
 # Use a non-root user (see shared/scripts/setup-server.sh)
 VPS_USER=deploy