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/i18n/strings_de.g.dart

@@ -199,7 +199,6 @@ class _Translations$common$de extends Translations$common$en {
 	@override String get delete => 'Löschen';
 	@override String get edit => 'Bearbeiten';
 	@override String get type => 'Typ';
-	@override String get comingSoon => 'Bald';
 	@override String get offline => 'Offline - Teilen ist unterbrochen';
 }
 
@@ -1461,7 +1460,6 @@ extension on TranslationsDe {
 			'common.delete' => 'Löschen',
 			'common.edit' => 'Bearbeiten',
 			'common.type' => 'Typ',
-			'common.comingSoon' => 'Bald',
 			'common.offline' => 'Offline - Teilen ist unterbrochen',
 			'home.tagline' => 'Teile und baue lokale Samen an',
 			'home.openMarket' => 'Markt',
@@ -1926,9 +1924,9 @@ extension on TranslationsDe {
 			'sale.add' => 'Verkauf notieren',
 			'sale.empty' => 'Noch keine Verkäufe. Notiere hier, was du verkaufst oder kaufst.',
 			'sale.iSold' => 'Ich verkaufte',
+			'sale.iBought' => 'Ich kaufte',
 			_ => null,
 		} ?? switch (path) {
-			'sale.iBought' => 'Ich kaufte',
 			'sale.direction' => 'Verkauft oder gekauft?',
 			'sale.counterparty' => 'Mit wem?',
 			'sale.counterpartyHint' => 'Eine Person oder ein Kollektiv (optional)',