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/admin/about/page.tsx

@@ -0,0 +1,41 @@
+"use client";
+
+import { SectionEditor } from "../_components/SectionEditor";
+import { InputField, TextareaField } from "../_components/FormField";
+import { ArrayEditor } from "../_components/ArrayEditor";
+
+interface AboutData {
+  title: string;
+  paragraphs: string[];
+}
+
+export default function AboutEditorPage() {
+  return (
+    <SectionEditor<AboutData> sectionKey="about" title="О студии">
+      {(data, update) => (
+        <>
+          <InputField
+            label="Заголовок секции"
+            value={data.title}
+            onChange={(v) => update({ ...data, title: v })}
+          />
+          <ArrayEditor
+            label="Параграфы"
+            items={data.paragraphs}
+            onChange={(paragraphs) => update({ ...data, paragraphs })}
+            renderItem={(text, _i, updateItem) => (
+              <TextareaField
+                label={`Параграф`}
+                value={text}
+                onChange={updateItem}
+                rows={3}
+              />
+            )}
+            createItem={() => ""}
+            addLabel="Добавить параграф"
+          />
+        </>
+      )}
+    </SectionEditor>
+  );
+}