feat(fase-12): features store + integration coverage

Adds the client mirror of section_enabled():

  * SectionKey / FeatureFlags types + SECTION_KEYS constant in
    @colectivo/types so the SQL and TS sides share the same vocabulary.
  * apps/web/src/lib/stores/features.ts:
      - currentUserFeatures      writable, loaded by root layout
      - enabledSections          derived map { section → boolean }
      - enabledSectionList       ordered SectionKey[] visible to UI;
                                 forces 'lists' if every layer says OFF
      - setUserFeature           per-user toggle (optimistic + rollback)
      - setCollectiveFeature     admin toggle (rolls back on RLS deny)
      - realtime subscriptions   one channel for the user's row, one
                                 for the active collective; rebuilt
                                 whenever those targets change

Wired into the root layout: loadCurrentUserFeatures runs from inside
the existing setTimeout-deferred onAuthStateChange callback (the
documented gotcha), and currentCollective.subscribe drives
subscribeCollectiveFeatures so collective switches re-target the
realtime filter without an extra auth event.

loadUserCollectives + every other place that fabricates a Collective
object now carries feature_flags (defaulting to {} for freshly created
collectives) so the Collective interface stays sound.

Integration spec (6 tests, SV-01..SV-06) covers default-ON, user-only,
collective-beats-user, RLS writes (user_update_own + collectives_update),
and per-collective isolation for a user in two collectives.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

apps/web/src/lib/components/CreateCollectiveModal.svelte

@@ -30,7 +30,11 @@
 			error = rpcErr?.message ?? 'Failed to create collective.';
 			return;
 		}
-		const created = data as { id: string; name: string; emoji: string; created_at: string };
+		const created = {
+			...(data as { id: string; name: string; emoji: string; created_at: string }),
+			// New collective starts with no flags — every section ON by default.
+			feature_flags: {} as import('@colectivo/types').FeatureFlags
+		};
 		userCollectives.update((list) => [...list, created]);
 		currentCollective.set(created);
 		localStorage.setItem('activeCollectiveId', created.id);