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/ui/home_screen_test.dart

@@ -2,15 +2,18 @@ import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:tane/app.dart';
 import 'package:tane/i18n/strings.g.dart';
+import 'package:tane/services/sharing_switch.dart';
 
 import '../support/test_support.dart';
 
 void main() {
-  Widget app(db) => TranslationProvider(
+  /// The everyday case: the social layer exists and sharing is already on.
+  Widget app(db, {SharingSwitch? sharing}) => TranslationProvider(
     child: TaneApp(
       repository: newTestRepository(db),
       species: newTestSpeciesRepository(db),
       onboarding: newTestOnboardingStore(),
+      sharing: sharing ?? newTestSharingSwitch(),
     ),
   );
 
@@ -24,7 +27,8 @@ void main() {
 
     expect(find.text('Your inventory'), findsOneWidget);
     expect(find.text('Market'), findsOneWidget);
-    expect(find.text('COMING SOON'), findsOneWidget); // market is Block 2
+    // "Coming soon" is gone for good: everything on this screen is built.
+    expect(find.textContaining('COMING SOON'), findsNothing);
 
     // Redesign copy: tagline + the two destination subtitles.
     expect(find.text('Share and grow local seeds'), findsOneWidget);
@@ -49,8 +53,9 @@ void main() {
     await tester.tap(find.byIcon(Icons.menu)); // hamburger
     await tester.pumpAndSettle();
 
-    // Social destinations are shown but disabled.
+    // With sharing on, the social destinations are live.
     expect(find.text('Your profile'), findsOneWidget);
+    expect(find.byIcon(Icons.lock_outline), findsNothing);
 
     await tester.tap(find.text('Inventory')); // active drawer item
     await tester.pumpAndSettle();
@@ -84,6 +89,62 @@ void main() {
     await disposeTree(tester);
   });
 
+  testWidgets('sharing off: market stays open, the rest wears a padlock',
+      (tester) async {
+    LocaleSettings.setLocaleSync(AppLocale.en);
+    final db = newTestDatabase();
+    addTearDown(db.close);
+
+    await tester.pumpWidget(
+      app(db, sharing: newTestSharingSwitch(enabled: false)),
+    );
+    await tester.pumpAndSettle();
+
+    // The market card is the door into sharing, so it must stay reachable.
+    expect(find.byKey(const Key('home.market')), findsOneWidget);
+
+    await tester.tap(find.byIcon(Icons.menu));
+    await tester.pumpAndSettle();
+
+    // Market live; the social entries are visible but padlocked — and tapping
+    // one invites the person in rather than doing nothing.
+    expect(find.text('Your profile'), findsOneWidget);
+    expect(find.byIcon(Icons.lock_outline), findsWidgets);
+
+    await tester.tap(find.text('Your profile'));
+    await tester.pumpAndSettle();
+    expect(find.text('This wakes up when you start sharing'), findsOneWidget);
+    await disposeTree(tester);
+  });
+
+  testWidgets('no social layer: the market and its friends are not drawn',
+      (tester) async {
+    LocaleSettings.setLocaleSync(AppLocale.en);
+    final db = newTestDatabase();
+    addTearDown(db.close);
+
+    await tester.pumpWidget(
+      TranslationProvider(
+        child: TaneApp(
+          repository: newTestRepository(db),
+          species: newTestSpeciesRepository(db),
+          onboarding: newTestOnboardingStore(),
+        ),
+      ),
+    );
+    await tester.pumpAndSettle();
+
+    // Nothing here can be switched on, so it is hidden rather than teased.
+    expect(find.byKey(const Key('home.market')), findsNothing);
+    expect(find.text('Your inventory'), findsOneWidget);
+
+    await tester.tap(find.byIcon(Icons.menu));
+    await tester.pumpAndSettle();
+    expect(find.text('Your profile'), findsNothing);
+    expect(find.byIcon(Icons.lock_outline), findsNothing);
+    await disposeTree(tester);
+  });
+
   testWidgets('drawer Settings opens the settings screen', (tester) async {