fix(auth+pwa): resolve stuck-loading race condition and PWA build error

Auth: `getSession()` in a SvelteKit load function races with Supabase's async
localStorage init and can return null for a valid session, triggering a spurious
login() redirect. Moved auth redirect to (app)/+layout.svelte via $effect, which
correctly waits for onAuthStateChange to fire. Also fixed collective data never
loading on page refresh (INITIAL_SESSION event, not SIGNED_IN).

PWA: SvelteKit blocks Workbox imports in src/service-worker.ts, preventing
@vite-pwa/sveltekit from finding the compiled SW output. Switched to generateSW
strategy (plugin auto-generates the SW). Custom SW deferred to Fase 2b using
src/sw.ts (a filename SvelteKit does not intercept).

Docs: added gotchas 6–8 to CLAUDE.md covering both root causes so they are not
reintroduced. Updated plan/fase-2b with the sw.ts workaround note.

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

plan/fase-2b-realtime-modo-compra.md

@@ -17,9 +17,11 @@
 - [ ] Esquema local en IndexedDB: `pending_ops`, `listas_cache`, `items_cache`
 - [ ] Cola de operaciones pendientes: cada mutación se escribe en `pending_ops` antes de llamar a Supabase
 - [ ] Service Worker (`@vite-pwa/sveltekit` + Workbox):
+  - **NOTA TÉCNICA:** SvelteKit intercepta `src/service-worker.ts` y bloquea imports externos (solo permite `$service-worker` y `$env/static/public`). Para usar Workbox con `injectManifest` hay que nombrar el fichero `src/sw.ts` (SvelteKit no lo reconoce como SW) y configurar `strategies: 'injectManifest'`, `filename: 'sw.ts'` en `vite.config.ts`. Actualmente el plugin usa `generateSW` (SW auto-generado, precaching básico).
   - Cache-first para assets estáticos
   - Network-first con fallback a caché para rutas de la app
   - Background sync para flush de `pending_ops` al recuperar conexión
+  - Fallback manual `online` event (Safari no soporta Background Sync API)
 - [ ] Fallback manual de sync: al detectar `online` en el evento del navegador, flush de la cola (cubre Safari que no soporta Background Sync API)
 - [ ] Indicador de estado de sincronización en la UI: `sincronizado | sincronizando | sin conexión`