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/lib/db.js

@@ -362,6 +362,24 @@ export async function setThemeSession(curriculumVersionId, weekNumber, theme, co
   );
 }
 
+/**
+ * Return every cached theme session, newest first. Used by the admin
+ * Content panel to surface what's been generated across all weeks.
+ */
+export async function getAllThemeSessions() {
+  try {
+    return await pb.collection('theme_sessions').getFullList({ sort: '-updated' });
+  } catch { return []; }
+}
+
+/**
+ * Delete a single theme session record by PocketBase id.
+ */
+export async function deleteThemeSession(id) {
+  try { return await pb.collection('theme_sessions').delete(id); }
+  catch { return null; }
+}
+
 /**
  * Has the user completed the theme session for a given personal week?
  * Returns the completion record, or null.