feat: admin panel with SQLite, auth, and calendar-style schedule editor

Complete admin panel for content management:
- SQLite database with better-sqlite3, seed script from content.ts
- Simple password auth with HMAC-signed cookies (Edge + Node compatible)
- 9 section editors: meta, hero, about, team, classes, schedule, pricing, FAQ, contact
- Team CRUD with image upload and drag reorder
- Schedule editor with Google Calendar-style visual timeline (colored blocks, overlap detection, click-to-add)
- All public components refactored to accept data props from DB (with fallback to static content)
- Middleware protecting /admin/* and /api/admin/* routes

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/app/not-found.tsx

@@ -1,13 +1,21 @@
 import { Button } from "@/components/ui/Button";
+import { Header } from "@/components/layout/Header";
+import { Footer } from "@/components/layout/Footer";
 
 export default function NotFound() {
   return (
-    <div className="flex min-h-[60vh] flex-col items-center justify-center px-4 text-center">
-      <h1 className="font-display text-6xl font-bold">404</h1>
-      <p className="body-text mt-4 text-lg">Страница не найдена</p>
-      <div className="mt-8">
-        <Button href="/">На главную</Button>
-      </div>
-    </div>
+    <>
+      <Header />
+      <main>
+        <div className="flex min-h-[60vh] flex-col items-center justify-center px-4 text-center">
+          <h1 className="font-display text-6xl font-bold">404</h1>
+          <p className="body-text mt-4 text-lg">Страница не найдена</p>
+          <div className="mt-8">
+            <Button href="/">На главную</Button>
+          </div>
+        </div>
+      </main>
+      <Footer />
+    </>
   );
 }