feat: enhance theme handling and layout components for Telegram Mini App

- Updated theme resolution logic to utilize a shared inline script for consistent theme application across routes.
- Introduced `AppShell` and `ReadyGate` components to manage app readiness and theme synchronization, improving user experience.
- Enhanced `GlobalError` and `NotFound` pages with a unified full-screen layout for better accessibility and visual consistency.
- Refactored CSS to implement safe area insets for sticky headers and content safety, ensuring proper layout on various devices.
- Added unit tests for new functionality and improved existing tests for better coverage and reliability.

webapp-next/src/app/admin/page.tsx

@@ -6,8 +6,10 @@
 
 "use client";
 
+import { useEffect } from "react";
 import { useAdminPage, AdminDutyList, ReassignSheet } from "@/components/admin";
 import { useTranslation } from "@/i18n/use-translation";
+import { useAppStore } from "@/store/app-store";
 import { AccessDeniedScreen } from "@/components/states/AccessDeniedScreen";
 import { LoadingState } from "@/components/states/LoadingState";
 import { ErrorState } from "@/components/states/ErrorState";
@@ -20,6 +22,12 @@ const PAGE_WRAPPER_CLASS =
 export default function AdminPage() {
   const { t, monthName } = useTranslation();
   const admin = useAdminPage();
+  const setAppContentReady = useAppStore((s) => s.setAppContentReady);
+
+  // Signal ready so Telegram hides loader when opening /admin directly.
+  useEffect(() => {
+    setAppContentReady(true);
+  }, [setAppContentReady]);
 
   if (!admin.isAllowed) {
     return (
@@ -57,7 +65,7 @@ export default function AdminPage() {
 
   return (
     <div className={PAGE_WRAPPER_CLASS}>
-      <header className="sticky top-0 z-10 flex flex-col items-center border-b border-border bg-background py-3">
+      <header className="sticky top-[var(--app-safe-top)] z-10 flex flex-col items-center border-b border-border bg-background py-3">
         <div className="flex w-full items-center justify-between px-1">
           <Button
             type="button"