feat(progress): associate progress with logged-in user for cross-device sync

Add optional user_id field to the progress collection. When a user is
authenticated, progress is keyed by user_id instead of session_id, making
it portable across devices and browsers. Anonymous reading still works via
session_id as before.

On login or registration, any progress accumulated under the anonymous
session is merged into the user's account (most-recent timestamp wins),
so chapters read before logging in are not lost.

- scripts/pb-init.sh: add user_id field to progress collection schema
- scraper/internal/storage/pocketbase.go: add user_id to EnsureCollections
- ui/src/lib/server/pocketbase.ts: Progress interface gains user_id;
  getProgress/allProgress/setProgress accept optional userId and query/write
  by user_id when present; add mergeSessionProgress() helper
- ui/src/routes/login/+page.server.ts: call mergeSessionProgress after
  successful login and registration (fire-and-forget, non-fatal)
- ui/src/routes/api/progress/+server.ts: pass locals.user?.id to setProgress
- ui/src/routes/books/+page.server.ts: pass locals.user?.id to allProgress
- ui/src/routes/books/[slug]/+page.server.ts: pass locals.user?.id to getProgress

scraper/internal/storage/pocketbase.go

@@ -325,6 +325,7 @@ func (s *PocketBaseStore) EnsureCollections(ctx context.Context) error {
 			"type": "base",
 			"fields": []map[string]interface{}{
 				{"name": "session_id", "type": "text", "required": true},
+				{"name": "user_id", "type": "text"},
 				{"name": "slug", "type": "text", "required": true},
 				{"name": "chapter", "type": "number"},
 				{"name": "updated", "type": "date"},