feat: hero video management — diagonal split, mobile single video, admin editor

- Hero: diagonal 3-video split on desktop, single center video on mobile (CSS breakpoint)
- Videos render client-side only to avoid hydration mismatch
- Loading overlay (z-5) hides buffering without blocking text content
- Admin hero editor: 3 fixed slots (left/center/right) with upload, preview, remove
- Center slot marked as main (used on mobile), save blocked until all 3 filled
- Upload API: supports MP4/WebM (50MB) in hero folder alongside existing image uploads
- Added videos field to hero type and fallback data

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/data/content.ts

@@ -12,6 +12,7 @@ export const siteContent: SiteContent = {
       "Открой для себя яркий, завораживающий и незабываемый мир танцев на пилоне!",
     ctaText: "Записаться",
     ctaHref: "#contact",
+    videos: ["/video/ira.mp4", "/video/nadezda.mp4", "/video/nastya-2.mp4"],
   },
   about: {
     title: "О нас",