fix: HIGH priority — scroll debounce, timing-safe auth, a11y, error logging, cleanup dead modals

- Header: throttle scroll handler via requestAnimationFrame (was firing 60+/sec)
- Auth: use crypto.timingSafeEqual for password and token signature comparison
- A11y: add role="dialog", aria-modal, aria-label to all modals (SignupModal, NewsModal, TeamProfile lightbox)
- A11y: add aria-label to close buttons, menu toggle (with aria-expanded), floating CTA
- A11y: add aria-label to MC Instagram buttons
- Error logging: add console.error with route names to all API catch blocks (admin + public)
- Fix open-day-register error leak (was returning raw DB error to client)
- Fix MasterClasses key={index} → key={item.title}
- Delete 3 unused modal components (BookingModal, MasterClassSignupModal, OpenDaySignupModal) — replaced by unified SignupModal

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

src/app/api/admin/open-day/route.ts

@@ -27,7 +27,8 @@ export async function POST(request: NextRequest) {
     }
     const id = createOpenDayEvent(body);
     return NextResponse.json({ ok: true, id });
-  } catch {
+  } catch (err) {
+    console.error("[admin/open-day] error:", err);
     return NextResponse.json({ error: "Internal error" }, { status: 500 });
   }
 }
@@ -39,7 +40,8 @@ export async function PUT(request: NextRequest) {
     const { id, ...data } = body;
     updateOpenDayEvent(id, data);
     return NextResponse.json({ ok: true });
-  } catch {
+  } catch (err) {
+    console.error("[admin/open-day] error:", err);
     return NextResponse.json({ error: "Internal error" }, { status: 500 });
   }
 }