Implement platform settings (Stage 9)

- Wire settings to actual platform behavior: maintenance mode, registration toggle,
  notification emails, curator feedback emails, email verification flag
- Add logo (logoUrl, showLogo) and social network links (YouTube, VK, Telegram) settings
- Show logo + school name dynamically in student layout header
- Add footer to student layout with org requisites and social links
- Register page: read settings server-side, validate terms checkbox with legal links
- Login page: show notice when redirected from closed registration
- Settings form: add Logo and Social Networks sections

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

src/lib/settings.ts

@@ -33,6 +33,15 @@ export const SETTINGS_DEFAULTS = {
   curatorCanAnswerQuestions: "true",
   curatorCanSeeStudents: "true",
 
+  // Logo
+  logoUrl: "",
+  showLogo: "true",
+
+  // Social networks
+  socialYoutube: "",
+  socialVk: "",
+  socialTelegram: "",
+
   // Code injection
   headCode: "",
   bodyCode: "",