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/sharing_invite_sheet.dart

@@ -0,0 +1,123 @@
+import 'package:flutter/material.dart';
+
+import '../i18n/strings.g.dart';
+import '../services/onboarding_store.dart';
+import '../services/sharing_switch.dart';
+import 'market_gate.dart';
+import 'theme.dart';
+
+/// Shown when someone taps a social entry (chat, profile, favourites, your
+/// people) while sharing is still off.
+///
+/// The alternative was hiding those entries until sharing is on, but then
+/// nobody would ever discover that Tane does any of it. So they stay in the
+/// drawer, quiet, and tapping one explains what they are and offers to switch
+/// them on — with the community rules, which is the same single consent step
+/// the market uses. Returns true when sharing ended up on.
+Future<bool> showSharingInvite(
+  BuildContext context, {
+  required OnboardingStore onboarding,
+  required SharingSwitch sharing,
+}) async {
+  final wants = await showModalBottomSheet<bool>(
+    context: context,
+    isScrollControlled: true,
+    builder: (_) => const SharingInviteSheet(),
+  );
+  if (wants != true || !context.mounted) return false;
+  return ensureMarketRulesAccepted(context, onboarding, sharing: sharing);
+}
+
+/// The invitation itself: what lights up when you join, and the plain fact that
+/// it needs a connection.
+class SharingInviteSheet extends StatelessWidget {
+  const SharingInviteSheet({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    final t = context.t;
+    final theme = Theme.of(context);
+    return SafeArea(
+      child: SingleChildScrollView(
+        padding: EdgeInsets.only(
+          left: 20,
+          right: 20,
+          top: 24,
+          bottom: 16 + MediaQuery.of(context).viewInsets.bottom,
+        ),
+        child: Column(
+          crossAxisAlignment: CrossAxisAlignment.start,
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            Text(
+              t.sharingInvite.title,
+              style: theme.textTheme.titleLarge?.copyWith(
+                color: seedOnSurface,
+                fontWeight: FontWeight.w600,
+              ),
+            ),
+            const SizedBox(height: 12),
+            _Perk(Icons.chat_bubble_outline, t.sharingInvite.perkChat),
+            _Perk(Icons.favorite_border, t.sharingInvite.perkFavorites),
+            _Perk(Icons.group_outlined, t.sharingInvite.perkPeople),
+            const SizedBox(height: 12),
+            Text(
+              t.sharingInvite.networkNote,
+              style: theme.textTheme.bodySmall?.copyWith(
+                color: seedMuted,
+                height: 1.4,
+              ),
+            ),
+            const SizedBox(height: 12),
+            Row(
+              mainAxisAlignment: MainAxisAlignment.end,
+              children: [
+                TextButton(
+                  key: const Key('sharingInvite.notNow'),
+                  onPressed: () => Navigator.of(context).pop(false),
+                  child: Text(t.sharingInvite.notNow),
+                ),
+                const SizedBox(width: 8),
+                FilledButton(
+                  key: const Key('sharingInvite.start'),
+                  onPressed: () => Navigator.of(context).pop(true),
+                  child: Text(t.sharingInvite.start),
+                ),
+              ],
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}
+
+class _Perk extends StatelessWidget {
+  const _Perk(this.icon, this.text);
+
+  final IconData icon;
+  final String text;
+
+  @override
+  Widget build(BuildContext context) {
+    return Padding(
+      padding: const EdgeInsetsDirectional.only(bottom: 8),
+      child: Row(
+        crossAxisAlignment: CrossAxisAlignment.start,
+        children: [
+          Icon(icon, size: 18, color: seedGreen),
+          const SizedBox(width: 8),
+          Expanded(
+            child: Text(
+              text,
+              style: Theme.of(context).textTheme.bodyMedium?.copyWith(
+                color: seedOnSurface,
+                height: 1.35,
+              ),
+            ),
+          ),
+        ],
+      ),
+    );
+  }
+}