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/src/lib/useAuth.ts

@@ -0,0 +1,35 @@
+import { useState, useEffect } from "react";
+
+export interface AuthUser {
+  discordId: string;
+  username: string;
+  avatar: string | null;
+}
+
+interface AuthState {
+  loading: boolean;
+  user: AuthUser | null;
+}
+
+export function useAuth(): AuthState & { logout: () => Promise<void> } {
+  const [state, setState] = useState<AuthState>({ loading: true, user: null });
+
+  useEffect(() => {
+    fetch("/auth/me", { credentials: "same-origin" })
+      .then((r) => r.json())
+      .then((data: { authenticated: boolean; user?: AuthUser }) => {
+        setState({
+          loading: false,
+          user: data.authenticated ? data.user! : null,
+        });
+      })
+      .catch(() => setState({ loading: false, user: null }));
+  }, []);
+
+  const logout = async () => {
+    await fetch("/auth/logout", { method: "POST", credentials: "same-origin" });
+    setState({ loading: false, user: null });
+  };
+
+  return { ...state, logout };
+}