feat: ratings, shelves, sleep timer, EPUB export + fix TS errors

**Ratings (1–5 stars)**
- New `book_ratings` PB collection (session_id, user_id, slug, rating)
- `getBookRating`, `getBookAvgRating`, `setBookRating` in pocketbase.ts
- GET/POST /api/ratings/[slug] API route
- StarRating.svelte component with hover, animated stars, avg display
- Star rating shown on book detail page (desktop + mobile)

**Plan-to-Read shelf**
- `shelf` field added to `user_library` (reading/plan_to_read/completed/dropped)
- `updateBookShelf`, `getShelfMap` in pocketbase.ts
- PATCH /api/library/[slug] for shelf updates
- Shelf selector dropdown on book detail page (only when saved)
- Shelf tabs on library page to filter by category

**Sleep timer**
- `sleepUntil` state added to AudioStore
- Layout handles timer lifecycle (survives chapter navigation)
- Cycles Off → 15m → 30m → 45m → 60m → Off
- Shows live countdown in AudioPlayer when active

**EPUB export**
- Go backend: GET /api/export/{slug}?from=N&to=N
- Generates valid EPUB2 zip (mimetype uncompressed, OPF, NCX, XHTML chapters)
- Markdown → HTML via goldmark
- SvelteKit proxy at /api/export/[slug]
- Download button on book detail page (only when in library)

**Fix TS errors**
- discover/+page.svelte: currentBook possibly undefined (use {@const book})
- cardEl now $state for reactive binding

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/pb-init-v3.sh

@@ -267,6 +267,14 @@ create "discovery_votes" '{
     {"name":"action",    "type":"text","required":true}
   ]}'
 
+create "book_ratings" '{
+  "name":"book_ratings","type":"base","fields":[
+    {"name":"session_id","type":"text",  "required":true},
+    {"name":"user_id",  "type":"text"},
+    {"name":"slug",     "type":"text",   "required":true},
+    {"name":"rating",   "type":"number", "required":true}
+  ]}'
+
 # ── 5. Field migrations (idempotent — adds fields missing from older installs) ─
 add_field "scraping_tasks" "heartbeat_at"           "date"
 add_field "audio_jobs"     "heartbeat_at"           "date"
@@ -282,5 +290,6 @@ add_field "app_users"      "oauth_provider"         "text"
 add_field "app_users"      "oauth_id"               "text"
 add_field "app_users"      "polar_customer_id"      "text"
 add_field "app_users"      "polar_subscription_id"  "text"
+add_field "user_library"   "shelf"                  "text"
 
 log "done"