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/components/AppShell.tsx

@@ -0,0 +1,17 @@
+/**
+ * App shell: wraps children with ReadyGate so any route can trigger miniAppReady().
+ * Rendered inside TelegramProvider so theme and SDK are available.
+ */
+
+"use client";
+
+import { ReadyGate } from "@/components/ReadyGate";
+
+export function AppShell({ children }: { children: React.ReactNode }) {
+  return (
+    <>
+      <ReadyGate />
+      {children}
+    </>
+  );
+}