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/services/social_settings_test.dart

@@ -101,4 +101,34 @@ void main() {
       {SocialSettings.offerKey('ab' * 32, 'tomate-1')},
     );
   });
+
+  group('sharing opt-in', () {
+    test('starts unanswered, then round-trips', () async {
+      expect(await settings.sharingEnabled(), isNull);
+      await settings.setSharingEnabled(true);
+      expect(await settings.sharingEnabled(), isTrue);
+      await settings.setSharingEnabled(false);
+      expect(await settings.sharingEnabled(), isFalse);
+    });
+
+    test('an install that had been through the intro keeps sharing on',
+        () async {
+      // The upgrade path that must not regress: these people were on a build
+      // that connected at launch, so they keep messaging, sync and alerts.
+      expect(await settings.migrateSharingEnabled(introSeen: true), isTrue);
+      expect(await settings.sharingEnabled(), isTrue);
+    });
+
+    test('a fresh install starts offline', () async {
+      expect(await settings.migrateSharingEnabled(introSeen: false), isFalse);
+      expect(await settings.sharingEnabled(), isFalse);
+    });
+
+    test('migration never overwrites an answer already given', () async {
+      await settings.setSharingEnabled(false);
+      // Later launches see the intro as seen; the recorded "no" must survive.
+      expect(await settings.migrateSharingEnabled(introSeen: true), isFalse);
+      expect(await settings.sharingEnabled(), isFalse);
+    });
+  });
 }