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/App.jsx

@@ -7,6 +7,7 @@ import ChatLauncher from './components/chat/ChatLauncher'
 
 import Login from './pages/Login'
 import Onboarding from './pages/Onboarding'
+import OnboardingTrack from './pages/OnboardingTrack'
 import Dashboard from './pages/Dashboard'
 
 import Admin from './pages/Admin'
@@ -106,6 +107,10 @@ function App() {
         <Route path="/login" element={<Login />} />
         <Route path="/onboarding" element={<Onboarding />} />
         <Route path="/" element={<ProtectedRoute><Dashboard /></ProtectedRoute>} />
+        {/* Onboarding track is available to any logged-in user, regardless of
+            enrollment — hence skipEnrollmentGate (distinct from /onboarding, the
+            enrollment page). */}
+        <Route path="/onboarding-track" element={<ProtectedRoute skipEnrollmentGate><OnboardingTrack /></ProtectedRoute>} />
         <Route path="/learn" element={<ProtectedRoute><Leren /></ProtectedRoute>} />
         <Route path="/test" element={<ProtectedRoute><Testen /></ProtectedRoute>} />
         <Route path="/topic-test" element={<ProtectedRoute><TopicTest /></ProtectedRoute>} />