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/themeSessionService.js

@@ -0,0 +1,61 @@
+import * as db from './db';
+import { callLLM, cachedSystem } from './llm';
+import { EMIT_THEME_SESSION_TOOL } from './llmTools';
+
+const THEME_SESSION_SYSTEM = `You are an expert learning content writer for Respellion, an internal IT company.
+You produce theme-level weekly learning sessions for the perpetual 26-week curriculum.
+A weekly session is a single, coherent piece of learning that covers every topic the curriculum has slotted for that week's theme — not a list of separate articles.
+Write in clear, professional English. Each topic section must be detailed (at least four sentences) and include a concrete workplace example or application.
+Emit content only through the emit_theme_session tool.`;
+
+function buildUserPrompt(weekContent) {
+  const topicLines = weekContent.topics
+    .map((t, idx) => `${idx + 1}. id=${t.id} | label="${t.label}" | type=${t.type} | description=${t.description}`)
+    .join('\n');
+
+  return `Theme: ${weekContent.theme}
+Week ${weekContent.weekNumber} of the 26-week curriculum.
+Estimated duration: ${weekContent.estimatedDuration || 30} minutes.
+Rationale for this week's position: ${weekContent.rationale || '—'}
+
+Topics in this theme this week (you MUST cover every one of them, one section each, reusing the exact topic_id):
+${topicLines}
+
+Produce a detailed theme session that synthesises these topics into a single coherent learning experience. Cover every topic in its own section, finish with a "how these connect" paragraph, and at least 3 takeaways for the theme.`;
+}
+
+/**
+ * Get the cached theme session for the active curriculum version + week,
+ * or generate one with the LLM and cache it.
+ *
+ * @param {string} curriculumVersionId — id of the active curriculum_versions row
+ * @param {object} weekContent — output of getCurrentWeekContent: { weekNumber, theme, topics[], estimatedDuration, rationale }
+ * @param {object} [opts]
+ * @param {boolean} [opts.force=false] — regenerate even if cached
+ */
+export async function getOrGenerateThemeSession(curriculumVersionId, weekContent, { force = false } = {}) {
+  if (!curriculumVersionId) throw new Error('Theme session requires an active curriculum version.');
+  if (!weekContent || !Array.isArray(weekContent.topics) || weekContent.topics.length === 0) {
+    throw new Error('Theme session requires at least one topic for the week.');
+  }
+
+  if (!force) {
+    const cached = await db.getThemeSession(curriculumVersionId, weekContent.weekNumber);
+    if (cached) return cached;
+  }
+
+  const result = await callLLM({
+    task: 'theme_session.generate',
+    tier: 'standard',
+    system: cachedSystem(THEME_SESSION_SYSTEM),
+    user: buildUserPrompt(weekContent),
+    tools: [EMIT_THEME_SESSION_TOOL],
+    toolChoice: { type: 'tool', name: EMIT_THEME_SESSION_TOOL.name },
+    maxTokens: 8192,
+  });
+
+  const emitted = result.toolUses[0]?.input;
+  if (!emitted) throw new Error('AI did not return a theme session. Please try again.');
+
+  return db.setThemeSession(curriculumVersionId, weekContent.weekNumber, weekContent.theme, emitted);
+}