feat: theme-grouped pickers, theme-session content panel, theme/topic naming

- Admin Topic Quizzes (was Quizzes): topics are now grouped under
  collapsible theme headers with per-theme counts. Topics without a
  theme fall into a "No theme" bucket rendered last.
- /topic-test picker: eligible topics grouped by theme so learners can
  scan a curriculum theme and drill its topics on demand.
- Fix admin Theme Content panel (was Learning Content): it was empty
  because content moved to the theme_sessions collection in an earlier
  refactor while the panel still queried the legacy per-topic content
  table. Rewrites ContentManager to read db.getAllThemeSessions and
  render the emit_theme_session schema (title, intro, topic sections,
  connections, key takeaways). Adds db.getAllThemeSessions and
  db.deleteThemeSession.
- Disambiguate theme vs topic across the app: the weekly curriculum
  test is now "Theme Test" (nav, page heading, Dashboard card, button,
  explainer); the on-demand bank test stays "Topic Test". Admin nav
  also clarified to "Theme Content" / "Topic Quizzes".

No schema changes. 85/85 tests still pass, build green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/pages/Testen.jsx

@@ -180,10 +180,10 @@ const Testen = () => {
       <div className="p-4 md:p-8 max-w-2xl mx-auto">
         <div className="text-center py-12">
           <CheckSquare size={64} className="mx-auto text-teal/30 mb-6" />
-          <h1 className="text-3xl md:text-4xl font-bold text-teal mb-4">Weekly Test</h1>
+          <h1 className="text-3xl md:text-4xl font-bold text-teal mb-4">Theme Test</h1>
           <p className="text-fg-muted text-lg mb-2">Cycle {getCurriculumCycle(weekNumber)} · Week {getCurriculumWeek(weekNumber)} of 26</p>
           <p className="text-fg-muted mb-8 max-w-md mx-auto">
-            Test your knowledge with 5 AI-generated questions. You have 5 minutes to complete the test. Good luck!
+            Test your knowledge of this week's theme with 5 AI-generated questions. You have 5 minutes to complete the test. Good luck!
           </p>
 
           {error && (