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/llmSchemas.js

@@ -213,6 +213,22 @@ export const replaceTakeawaysPatchSchema = z.object({
   items: z.array(z.string().min(1)).min(1),
 });
 
+// ── Theme session schema ─────────────────────────────────────────────────────
+
+const themeSessionTopicSection = z.object({
+  topic_id: z.string().min(1),
+  label: z.string().min(1),
+  summary: z.string().min(1),
+});
+
+export const themeSessionSchema = z.object({
+  title: z.string().min(1),
+  intro: z.string().min(1),
+  topicSections: z.array(themeSessionTopicSection).min(1),
+  connections: z.string().min(1),
+  keyTakeaways: z.array(z.string().min(1)).min(3),
+});
+
 /**
  * Registry mapping known tool names to their input schemas. `callLLM`
  * consults this when the caller does not pass an explicit `toolSchemas`
@@ -235,4 +251,5 @@ export const toolSchemaRegistry = {
   add_section: addSectionPatchSchema,
   remove_section: removeSectionPatchSchema,
   replace_takeaways: replaceTakeawaysPatchSchema,
+  emit_theme_session: themeSessionSchema,
 };