feat(sharing): make going online opt-in, and show what it unlocks

Tane dialled its four default relays at launch, before anyone had asked
for anything — an F-Droid reviewer spotted it, and they were right. The
seed book needs no network at all, so the app should not have one until
the person joins the sharing side.

- SocialSettings gains a three-state `sharingEnabled`. `null` means
  "never asked", which is what lets `migrateSharingEnabled` keep an
  existing install exactly as it was: anyone past the intro was on a
  build that connected at launch, so they keep messaging, device sync
  and offer alerts. A fresh install starts fully offline.
- bootstrap only starts the shared connection when sharing is on. The
  inbox/sync/plantaré/alert listeners are untouched: they react to a
  session, and none arrives.
- SharingSwitch is the single place that moves the stored choice, the
  live connection and the flag the UI listens to, so they cannot drift.
- Agreeing to the community rules is the opt-in — one consent surface,
  reached from the market or from the drawer's invitation.
- SocialConnection.start is now idempotent and gains stop(), so turning
  sharing off goes offline immediately instead of at the next launch.
- The social drawer entries stay visible but padlocked while sharing is
  off; tapping one explains what wakes up and offers to join. Hiding
  them would have kept the tool a secret. "Coming soon" is gone for
  good — everything it labelled is built.

Covered by tests for the migration in both directions, start/stop
lifecycle, the gate turning sharing on, the invitation, and the drawer
in all three states (no social layer / off / on).

apps/app_seeds/test/support/test_support.dart

@@ -11,6 +11,8 @@ import 'package:tane/db/database.dart';
 import 'package:tane/i18n/strings.g.dart';
 import 'package:tane/security/secret_store.dart';
 import 'package:tane/services/onboarding_store.dart';
+import 'package:tane/services/sharing_switch.dart';
+import 'package:tane/services/social_settings.dart';
 import 'package:tane/app.dart' show materialLocaleFor;
 import 'package:tane/state/inventory_cubit.dart';
 import 'package:tane/state/variety_detail_cubit.dart';
@@ -59,6 +61,14 @@ OnboardingStore newTestOnboardingStore({bool introSeen = true}) {
   return OnboardingStore(store);
 }
 
+/// A [SharingSwitch] over in-memory storage and no relay connection, for
+/// screens that only care whether sharing is on. Defaults to on, which is what
+/// most screen tests want (the social entries live, nothing dialled).
+SharingSwitch newTestSharingSwitch({bool enabled = true}) => SharingSwitch(
+      settings: SocialSettings(InMemorySecretStore()),
+      enabled: enabled,
+    );
+
 /// Wraps [child] with the providers a screen expects (repository, inventory
 /// cubit) plus i18n and Material localizations, pinned to [locale].
 Widget wrapScreen({