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/App.jsx

@@ -50,7 +50,7 @@ const ProtectedRoute = ({ children, requireAdmin, skipEnrollmentGate }) => {
         <div className="hidden md:flex items-center gap-6 text-sm font-medium">
           <Link to="/" className="hover:text-accent-soft transition-colors flex items-center gap-2"><LayoutDashboard size={16}/> Dashboard</Link>
           <Link to="/learn" className="hover:text-accent-soft transition-colors flex items-center gap-2"><BookOpen size={16}/> Learn</Link>
-          <Link to="/test" className="hover:text-accent-soft transition-colors flex items-center gap-2"><CheckSquare size={16}/> Test</Link>
+          <Link to="/test" className="hover:text-accent-soft transition-colors flex items-center gap-2"><CheckSquare size={16}/> Theme Test</Link>
           <Link to="/topic-test" className="hover:text-accent-soft transition-colors flex items-center gap-2"><Target size={16}/> Topic Test</Link>
           <Link to="/leaderboard" className="hover:text-accent-soft transition-colors flex items-center gap-2"><Trophy size={16}/> Leaderboard</Link>
           {state.currentUser.role === 'admin' && (
@@ -78,7 +78,7 @@ const ProtectedRoute = ({ children, requireAdmin, skipEnrollmentGate }) => {
       <nav className="md:hidden fixed bottom-0 left-0 right-0 bg-paper border-t border-bg-warm flex justify-around items-center p-2 z-50">
           <Link to="/" className="flex flex-col items-center p-2 text-fg-muted hover:text-teal"><LayoutDashboard size={20}/><span className="text-[10px] mt-1 font-medium">Home</span></Link>
           <Link to="/learn" className="flex flex-col items-center p-2 text-fg-muted hover:text-teal"><BookOpen size={20}/><span className="text-[10px] mt-1 font-medium">Learn</span></Link>
-          <Link to="/test" className="flex flex-col items-center p-2 text-fg-muted hover:text-teal"><CheckSquare size={20}/><span className="text-[10px] mt-1 font-medium">Test</span></Link>
+          <Link to="/test" className="flex flex-col items-center p-2 text-fg-muted hover:text-teal"><CheckSquare size={20}/><span className="text-[10px] mt-1 font-medium">Theme</span></Link>
           <Link to="/topic-test" className="flex flex-col items-center p-2 text-fg-muted hover:text-teal"><Target size={20}/><span className="text-[10px] mt-1 font-medium">Topic</span></Link>
           <Link to="/leaderboard" className="flex flex-col items-center p-2 text-fg-muted hover:text-teal"><Trophy size={20}/><span className="text-[10px] mt-1 font-medium">Score</span></Link>
           {state.currentUser.role === 'admin' && (