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/lib/ui/home_screen.dart

@@ -2,6 +2,8 @@ import 'package:flutter/material.dart';
 import 'package:go_router/go_router.dart';
 
 import '../i18n/strings.g.dart';
+import '../services/onboarding_store.dart';
+import '../services/sharing_switch.dart';
 import 'app_drawer.dart';
 import 'seed_glyph.dart';
 import 'theme.dart';
@@ -9,14 +11,19 @@ import 'unread_badge.dart';
 
 /// The main menu (redesign screen 00): a sprout logo in a soft green disc over a
 /// faint seed-glyph watermark, with "Your inventory" as the primary green call
-/// to action and "Open market" (Block 2) as a disabled outlined card. The
-/// hamburger opens [AppDrawer].
+/// to action and "Open market" as an outlined card below it. The hamburger opens
+/// [AppDrawer].
+///
+/// The market card is always live when the social layer exists — it is the door
+/// people go through to join sharing — and simply isn't drawn when it doesn't.
 class HomeScreen extends StatelessWidget {
-  const HomeScreen({this.marketEnabled = false, super.key});
+  const HomeScreen({this.sharing, this.onboarding, super.key});
 
-  /// When the Block 2 social layer is wired, the market becomes a live
-  /// destination; otherwise it stays a disabled "coming soon" card.
-  final bool marketEnabled;
+  /// The sharing on/off switch, or null when there is no social layer at all.
+  final SharingSwitch? sharing;
+
+  /// Needed to run the community-rules step from the drawer's invitation.
+  final OnboardingStore? onboarding;
 
   @override
   Widget build(BuildContext context) {
@@ -34,7 +41,7 @@ class HomeScreen extends StatelessWidget {
           ),
         ),
       ),
-      drawer: AppDrawer(marketEnabled: marketEnabled),
+      drawer: AppDrawer(sharing: sharing, onboarding: onboarding),
       body: Stack(
         children: [
           const Positioned.fill(child: _SeedWatermark()),
@@ -99,17 +106,16 @@ class HomeScreen extends StatelessWidget {
                       subtitle: t.home.yourInventorySubtitle,
                       onTap: () => context.push('/inventory'),
                     ),
-                    const SizedBox(height: 16),
-                    _OutlinedMenuCard(
-                      key: const Key('home.market'),
-                      icon: Icons.storefront_outlined,
-                      label: t.home.openMarket,
-                      subtitle: t.home.openMarketSubtitle,
-                      tag: marketEnabled ? null : t.common.comingSoon,
-                      onTap: marketEnabled
-                          ? () => context.push('/market')
-                          : null,
-                    ),
+                    if (sharing != null) ...[
+                      const SizedBox(height: 16),
+                      _OutlinedMenuCard(
+                        key: const Key('home.market'),
+                        icon: Icons.storefront_outlined,
+                        label: t.home.openMarket,
+                        subtitle: t.home.openMarketSubtitle,
+                        onTap: () => context.push('/market'),
+                      ),
+                    ],
                   ],
                 ),
               ),
@@ -175,14 +181,12 @@ class _PrimaryMenuCard extends StatelessWidget {
   }
 }
 
-/// A disabled Block-2 destination: an outlined white card with a green-disc
-/// icon and a "soon" tag.
+/// A secondary destination: an outlined white card with a green-disc icon.
 class _OutlinedMenuCard extends StatelessWidget {
   const _OutlinedMenuCard({
     required this.icon,
     required this.label,
     required this.subtitle,
-    this.tag,
     this.onTap,
     super.key,
   });
@@ -191,9 +195,6 @@ class _OutlinedMenuCard extends StatelessWidget {
   final String label;
   final String subtitle;
 
-  /// A small trailing tag (e.g. "coming soon"); omitted for live cards.
-  final String? tag;
-
   /// When set, the card is tappable; otherwise it reads as disabled.
   final VoidCallback? onTap;
 
@@ -221,17 +222,7 @@ class _OutlinedMenuCard extends StatelessWidget {
               subtitleColor: seedMuted,
             ),
           ),
-          if (tag != null)
-            Text(
-              tag!.toUpperCase(),
-              style: const TextStyle(
-                color: Color(0xFFB3BDA8),
-                fontSize: 11,
-                fontWeight: FontWeight.w500,
-                letterSpacing: 0.5,
-              ),
-            )
-          else if (onTap != null)
+          if (onTap != null)
             const Icon(Icons.chevron_right, color: seedMuted),
         ],
       ),