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/0005_wealthy_golden_guardian.sql

@@ -0,0 +1,13 @@
+CREATE TABLE "game_settings" (
+	"id" text PRIMARY KEY DEFAULT 'default' NOT NULL,
+	"leveling" jsonb NOT NULL,
+	"economy" jsonb NOT NULL,
+	"inventory" jsonb NOT NULL,
+	"lootdrop" jsonb NOT NULL,
+	"trivia" jsonb NOT NULL,
+	"moderation" jsonb NOT NULL,
+	"commands" jsonb DEFAULT '{}'::jsonb,
+	"system" jsonb DEFAULT '{}'::jsonb,
+	"created_at" timestamp with time zone DEFAULT now() NOT NULL,
+	"updated_at" timestamp with time zone DEFAULT now() NOT NULL
+);