feat: theme-level weekly sessions and theme-grouped knowledge library

- New theme_sessions + theme_session_completions PocketBase collections
- Generate a detailed per-week theme summary covering every topic in the
  week's curriculum slot via EMIT_THEME_SESSION_TOOL; cache per
  (curriculum_version, week_number)
- Replace Leren.jsx "This Week's Topic" card with "This Week's Theme
  Session" that opens the new ThemeSessionView; per-topic micro-learnings
  remain reachable as optional practice from inside the session
- Group the Knowledge Library by topic.theme, pin the current week's
  theme on top, and badge topics already covered by the active session
- Mark week complete when the user finishes reading the theme session

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

src/lib/db.js

@@ -242,6 +242,61 @@ export async function getLearnDone() { return false; }
 /** @deprecated learn_progress collection has been dropped. */
 export async function setLearnDone() { return null; }
 
+// ── Theme Sessions ──────────────────────────────────────────────────────────
+
+/**
+ * Look up the cached theme session for a given (curriculum_version, week_number).
+ * Returns null when no cache entry exists yet.
+ */
+export async function getThemeSession(curriculumVersionId, weekNumber) {
+  try {
+    const record = await pb.collection('theme_sessions').getFirstListItem(
+      `curriculum_version = "${curriculumVersionId}" && week_number = ${weekNumber}`
+    );
+    return record;
+  } catch {
+    return null;
+  }
+}
+
+/**
+ * Upsert the theme session for a given (curriculum_version, week_number).
+ */
+export async function setThemeSession(curriculumVersionId, weekNumber, theme, content) {
+  return pbUpsert(
+    'theme_sessions',
+    `curriculum_version = "${curriculumVersionId}" && week_number = ${weekNumber}`,
+    { theme, content },
+    { curriculum_version: curriculumVersionId, week_number: weekNumber, theme, content },
+  );
+}
+
+/**
+ * Has the user completed the theme session for a given personal week?
+ * Returns the completion record, or null.
+ */
+export async function getThemeSessionCompletion(userId, sessionWeek) {
+  try {
+    return await pb.collection('theme_session_completions').getFirstListItem(
+      `team_member_id = "${userId}" && session_week = ${sessionWeek}`
+    );
+  } catch {
+    return null;
+  }
+}
+
+/**
+ * Mark a theme session as completed for the user/week. Idempotent.
+ */
+export async function setThemeSessionCompletion(userId, themeSessionId, sessionWeek) {
+  return pbUpsert(
+    'theme_session_completions',
+    `team_member_id = "${userId}" && session_week = ${sessionWeek}`,
+    { theme_session_id: themeSessionId },
+    { team_member_id: userId, theme_session_id: themeSessionId, session_week: sessionWeek },
+  );
+}
+
 // ── Leaderboard ───────────────────────────────────────────────────────────────
 
 export async function getLeaderboard() {
@@ -484,6 +539,8 @@ export async function resetForSmokeTest({
     'topics',
     'micro_learnings',
     'micro_learning_completions',
+    'theme_session_completions',
+    'theme_sessions',
     'test_results',
   ];
   if (includeProgress) collections.push('leaderboard');