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>

shared/db/migrations/meta/_journal.json

@@ -36,6 +36,13 @@
       "when": 1770904612078,
       "tag": "0004_bored_kat_farrell",
       "breakpoints": true
+    },
+    {
+      "idx": 5,
+      "version": "7",
+      "when": 1771010684586,
+      "tag": "0005_wealthy_golden_guardian",
+      "breakpoints": true
     }
   ]
 }