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_ja.g.dart

@@ -68,7 +68,6 @@ class _Translations$common$ja extends Translations$common$en {
 	@override String get delete => '削除';
 	@override String get edit => '編集';
 	@override String get type => '種類';
-	@override String get comingSoon => '近日公開';
 	@override String get offline => 'オフラインです — 共有を一時停止しています';
 }
 
@@ -140,7 +139,6 @@ extension on TranslationsJa {
 			'common.delete' => '削除',
 			'common.edit' => '編集',
 			'common.type' => '種類',
-			'common.comingSoon' => '近日公開',
 			'common.offline' => 'オフラインです — 共有を一時停止しています',
 			'menu.tagline' => 'あなたの種子バンク',
 			'menu.inventory' => '在庫',