feat(admin): GitHub OAuth login, admin whitelist and Nuxt WebUI

Add an admin WebUI (Nuxt static SPA in admin/, served via the ASSETS
binding) for managing routes and viewing send logs. Access is gated by
GitHub OAuth plus an ADMIN_USER_IDS whitelist with cookie sessions
(admin-session.ts). Expose routes/logs CRUD API (admin-routes.ts), add a
discord-link mapping in token-store.ts, and mount admin routes with an
SPA assets fallback in the server.

admin/composables/useLogs.ts

@@ -0,0 +1,33 @@
+import type { SendRecord } from "~/types";
+
+export function useSendLogs() {
+  const logs = ref<SendRecord[]>([]);
+  const loading = ref(false);
+  const needLogin = ref(false);
+  const error = ref("");
+
+  async function load(limit = 50): Promise<void> {
+    loading.value = true;
+    error.value = "";
+    needLogin.value = false;
+    try {
+      const res = await fetch(`/admin/api/logs?limit=${limit}`, {
+        headers: { accept: "application/json" },
+        credentials: "same-origin",
+      });
+      if (res.status === 401) {
+        needLogin.value = true;
+        return;
+      }
+      if (!res.ok) throw new Error(`HTTP ${res.status}`);
+      const data = (await res.json()) as { logs?: SendRecord[] };
+      logs.value = data.logs ?? [];
+    } catch (err) {
+      error.value = err instanceof Error ? err.message : String(err);
+    } finally {
+      loading.value = false;
+    }
+  }
+
+  return { logs, loading, needLogin, error, load };
+}