feat(fase-2b): Realtime sync + offline queue + Modo Compra

Fase 2b closes the critical-path product differentiator. Two sessions on the
same list see each other's changes in real time, mutations survive network
drops and sync when reconnected, and a dedicated full-screen shopping view
optimises the in-store experience.

2b.1 Realtime
- `$lib/stores/realtimeSync.ts` wraps `postgres_changes` with an `applyItemEvent`
  reducer (INSERT/UPDATE/DELETE → next items[]) and a `subscribeToList` helper
  that awaits the SUBSCRIBED handshake before returning
- `/lists/[id]` subscribes in onMount, unsubscribes in onDestroy. Uses a
  `pendingTempIds` set to deduplicate own-mutation echoes against optimistic
  rows (matches by name+created_by+sort_order)
- Client-generated UUIDs for new inserts make the path idempotent: if the
  server response is lost and we retry, the second POST hits PK-conflict
  which the queue treats as success
- CHECKED section div now carries role="listitem" so Playwright locators
  follow the item across sections

2b.2 Offline queue
- `$lib/sync/queue.ts` — SyncQueue class backed by IndexedDB (idb), FIFO flush,
  MAX_ATTEMPTS=5 retry budget, PK-conflict-as-success short-circuit
- `$lib/sync/index.ts` — app-wide singleton, window.online listener flushes
  automatically, hydrateSyncState() at page load restores pendingOpsCount
- `$lib/stores/syncStatus.ts` — derived store (offline | syncing | synced)
  tracking navigator.onLine + queue depth
- SyncBanner component renders the offline/syncing indicator in the detail
  and session views
- handleAdd enqueues on failure instead of reverting, so an offline mutation
  keeps its optimistic row and syncs on reconnect

2b.3 Modo Compra
- `/lists/[id]/session/+page.svelte` — full-screen overlay (fixed inset-0
  z-50) that covers the sidebar while keeping the normal auth routing.
  56×56 toggles, flip animation shuffling items between TO BUY / CHECKED,
  Finish Shopping confirmation modal → completeList → goto('/lists')
- Link from `/lists/[id]` header (data-testid=start-session) with the
  new `list_start_session` message

Testing infra
- apps/web gets its own Vitest config (jsdom + fake-indexeddb/auto) and a
  new `pnpm test:unit` script. `just test-all` now chains pgTAP → integration
  → unit → e2e so a single command is the gate.
- packages/test-utils/tests/sync-queue.test.ts (placeholder scaffold) removed —
  replaced by `apps/web/src/lib/sync/queue.test.ts` co-located with the module

Totals: 103 tests green
  16 pgTAP
  59 Vitest integration (in packages/test-utils)
   6 Vitest unit (in apps/web — the SyncQueue)
  22 Playwright E2E (5 auth + 4 lists + 6 items + 2 realtime + 2 offline + 3 session)
   2 skipped (realtime-presence — upstream bug, unchanged)

Documented in plan/fase-2b and CLAUDE.md.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -8,7 +8,7 @@ This file provides guidance to Claude Code (claude.ai/code) when working with co
 
 ## Project Status
 
-**Fase 0 complete. Fase 1 complete. Fase 2a complete. Fase 2b — Realtime infra prep done, R-series (postgres_changes + isolation) green, presence blocked by upstream Realtime bug. Full test suite: 59 Vitest passed (+ 2 presence + 7 sync-queue skipped awaiting implementation/upstream fix) + 16 pgTAP + 15 Playwright (+ 7 E2E scaffolded for 2b UI) = 90 green. ✅**
+**Fase 0 complete. Fase 1 complete. Fase 2a complete. Fase 2b complete (Realtime + offline queue + Modo Compra). 103 tests green: 16 pgTAP + 59 Vitest integration + 6 Vitest unit + 22 Playwright. 2 skipped (Realtime presence — upstream bug in `supabase/realtime` `handle_out/3`, reactivate when fixed). ✅**
 
 - `README.md` — full development plan, confirmed tech stack, Justfile reference, and technical warnings
 - `analysis/analisis-funcional.md` — complete functional specification (domain model, use cases, data models, business rules)
@@ -73,6 +73,21 @@ just test-e2e            # Playwright E2E tests (requires just dev running)
 just test-e2e-headed     # Playwright with visible browser (debugging)
 ```
 
+### Fase 2b — what has been built
+
+- `supabase/migrations/007_realtime_publication.sql` — adds shopping_items + shopping_lists to `supabase_realtime` publication with `REPLICA IDENTITY FULL`
+- `apps/web/src/lib/stores/realtimeSync.ts` — `subscribeToList(listId, onEvent)` postgres_changes subscription helper; `applyItemEvent` reducer for INSERT/UPDATE/DELETE
+- `/lists/[id]/+page.svelte` — wired to realtimeSync with pendingTempIds echo-deduplication. Mutations use client-generated UUIDs so offline retries are idempotent (PK-conflict = success)
+- `apps/web/src/lib/sync/queue.ts` — `SyncQueue` class (IDB-backed pending_ops, FIFO flush, MAX_ATTEMPTS retry, PK-conflict-as-success)
+- `apps/web/src/lib/sync/index.ts` — app-wide SyncQueue singleton; `enqueueOp`, `hydrateSyncState`, auto-flush on `window.online`
+- `apps/web/src/lib/stores/syncStatus.ts` — derived store: `offline | syncing | synced` from `navigator.onLine` + `pendingOpsCount`
+- `apps/web/src/lib/components/SyncBanner.svelte` — renders the offline/syncing indicator; mounted in list detail + session views
+- `apps/web/src/routes/(app)/lists/[id]/session/+page.svelte` — Modo Compra full-screen view (fixed positioning overlays the sidebar), 56px toggles, flip animation, confirm modal, completeList → redirect
+- `apps/web/messages/*.json` — `sync_offline`, `sync_syncing`, `list_start_session`
+- `packages/test-utils/src/realtime-helpers.ts` — `subscribePostgresChanges(client, opts)` with `waitFor(predicate, ms)` — awaits SUBSCRIBED handshake, dedups events, leaks nothing between tests
+- Vitest unit harness in `apps/web/` — `vitest.config.ts` (jsdom env) + `vitest.setup.ts` (fake-indexeddb/auto) + `pnpm --filter @colectivo/web test:unit`
+- Realtime config gotchas documented in `project_realtime_config` memory: tenant name, DB_USER=supabase_admin, `SEED_SELF_HOST: "true"`, pre-created `realtime` schema, publication + REPLICA IDENTITY FULL
+
 ### Fase 2a — what has been built
 
 - `supabase/migrations/005_shopping.sql` — `shopping_lists`, `shopping_items`, RLS, `list_collective_id()` helper, pg_cron jobs (archive completed > 7d, purge deleted > 7d)