feat(admin): surface classroom feature toggle in tab-games

The feature_classroom_enabled flag was fully wired in backend
(classroom/sessions.js:11-14 returns 403 when '0', initialized in
legacy-migrate.js:870 to '1') but had no UI control — admin could
only flip it via direct SQL.

- adminController.updateFeatures: classroom was ALREADY in allowed list
- admin/sections/games.js: new toggle row with video icon added to GAME_FEATURES
- js/api.js hideDisabledFeatures: classroom path mapping added ('/classroom')

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

frontend/js/admin/sections/games.js

@@ -15,6 +15,7 @@
     { key: 'board',         label: 'Доска',             desc: 'Классная доска с объявлениями, постами и обсуждениями',             icon: 'layout-dashboard'},
     { key: 'biochem',       label: 'Биохимия',          desc: 'Молекулярный редактор, задачи на построение молекул и реакции',     icon: 'flask-conical'  },
     { key: 'live_quiz',     label: 'Живая викторина',  desc: 'Синхронная викторина в реальном времени для всего класса',          icon: 'radio'          },
+    { key: 'classroom',     label: 'Онлайн-уроки (classroom)', desc: 'Синхронные онлайн-уроки с доской и видео',                  icon: 'video'          },
   ];
 
   const FS_FEATURES = [

js/api.js

@@ -801,6 +801,7 @@ async function hideDisabledFeatures() {
     board:         ['/board'],
     biochem:       ['/biochem', '/biochem-library', '/biochem-reactions'],
     live_quiz:     ['/live-quiz'],
+    classroom:     ['/classroom'],
     exam9:         ['/exam9', '/exam9.html'],
     textbooks:     ['/textbooks', '/textbooks.html', '/textbook'],
   };