feat: 5-day theme-level onboarding track from the "New here?" card (#30)

A self-paced onboarding track that introduces a new employee to every KB
theme in breadth (not depth), so they grasp how Respellion works day to
day and week to week. Offered as a CTA inside the Dashboard "New here?"
explainer card; always available regardless of enrollment.

Design:
- Theme is the trackable unit; the 5 "days" are a read-time presentation
  grouping, so re-chunking never loses progress. Completion is stored per
  theme in onboarding_completions.
- Per-theme overview generated lazily on first open (fast-tier
  emit_onboarding_overview tool), cached in onboarding_overviews keyed by
  theme + a topics_fingerprint that triggers regeneration when the theme's
  topic set changes.
- Reachable via /onboarding-track using the existing skipEnrollmentGate
  prop, decoupled from the 26-week curriculum (distinct from /onboarding,
  the enrollment page).

Backend:
- pb_migrations/1781200000_created_onboarding.js: two collections with
  authenticated-only rules and unique indexes; TEXT team_member_id (no
  relation) per the post-#18/#27 convention. Mirrored in
  scripts/setup-pb-collections.mjs.
- src/lib/onboardingService.js: pure helpers (orderThemes,
  distributeThemesIntoDays, computeTopicsFingerprint,
  computeOnboardingProgress, buildOnboardingPlan) + generation + I/O.
- db.js onboarding helpers use pb.filter() bindings (theme is free text).
- LLM tool + Zod schema + registry + simulation stub.

Frontend:
- src/pages/OnboardingTrack.jsx (day list, per-theme overview, completion
  banner, progress ring/day bar).
- Dashboard "New here?" card CTA + X/5-days progress chip (hidden when the
  KB has no themes).

Docs: data-model, generation-spec (§D), frontend-spec updated.

Verified: 22 new unit tests (npm test 134/134), eslint clean on changed
files, npm run build OK, PocketBase v0.30.4 boot applies the migration
(collections + unique indexes + authed rules confirmed), and a backend
contract check (upsert idempotency, unique-index guard, special-char
theme filtering).

Closes #30

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/lib/llmTools.js

@@ -454,3 +454,39 @@ export const EMIT_THEME_SESSION_TOOL = {
   },
 };
 
+// ── Onboarding overview (breadth-first, one theme) ───────────────────────────
+
+export const EMIT_ONBOARDING_OVERVIEW_TOOL = {
+  name: 'emit_onboarding_overview',
+  description: 'Return a SHORT, breadth-first onboarding overview of ONE theme for a brand-new Respellion employee. This is a light introduction, NOT a deep lesson: what the theme is in plain language, why it matters for day-to-day and week-to-week work at Respellion, a few key points, and which topics belong to it. Keep it skimmable.',
+  input_schema: {
+    type: 'object',
+    properties: {
+      title: { type: 'string', description: 'Learner-facing theme title, 2–6 words.' },
+      what_it_is: { type: 'string', description: '1–2 plain-language sentences defining what this theme is about.' },
+      why_it_matters: { type: 'string', description: '1–3 sentences on why this theme matters for a new employee\'s daily and weekly work at Respellion.' },
+      key_points: {
+        type: 'array',
+        items: { type: 'string' },
+        minItems: 3,
+        maxItems: 5,
+        description: '3–5 short, concrete takeaways a newcomer should remember about this theme.',
+      },
+      topics_covered: {
+        type: 'array',
+        description: 'The topics that make up this theme. Reuse the exact topic_id you were given so the UI can link back.',
+        items: {
+          type: 'object',
+          properties: {
+            topic_id: { type: 'string', description: 'The id of the topic (must match one of the ids provided).' },
+            label: { type: 'string', description: 'The topic label, as provided.' },
+          },
+          required: ['topic_id', 'label'],
+        },
+        minItems: 1,
+      },
+    },
+    required: ['title', 'what_it_is', 'why_it_matters', 'key_points', 'topics_covered'],
+  },
+};
+