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/llmSchemas.js

@@ -229,6 +229,16 @@ export const themeSessionSchema = z.object({
   keyTakeaways: z.array(z.string().min(1)).min(3),
 });
 
+export const onboardingOverviewSchema = z.object({
+  title: z.string().min(1),
+  what_it_is: z.string().min(1),
+  why_it_matters: z.string().min(1),
+  key_points: z.array(z.string().min(1)).min(3).max(5),
+  topics_covered: z
+    .array(z.object({ topic_id: z.string().min(1), label: z.string().min(1) }))
+    .min(1),
+});
+
 /**
  * Registry mapping known tool names to their input schemas. `callLLM`
  * consults this when the caller does not pass an explicit `toolSchemas`
@@ -252,4 +262,5 @@ export const toolSchemaRegistry = {
   remove_section: removeSectionPatchSchema,
   replace_takeaways: replaceTakeawaysPatchSchema,
   emit_theme_session: themeSessionSchema,
+  emit_onboarding_overview: onboardingOverviewSchema,
 };