feat(block2): default relays + pick-from-list servers (works out of the box)

Feedback: the blank 'web address' field was unusable — nobody knows what to put,
so the market never worked at first launch.
- SocialSettings ships default public relays (nos.lol, relay.damus.io), applied
  automatically for new users so the market works day one. An explicitly empty
  choice still turns the network off (privacy). Exposure is minimal: offers are
  opt-in and carry only a coarse geohash.
- Sharing setup now shows the servers as a short PICK-LIST (checkboxes, pre-
  selected) instead of a blank field; a community's own server goes under a small
  'add another' escape hatch. Intro reworded — no need to know a community.
- Tests updated (defaults) and pinned offline (setRelayUrls([])) so widget tests
  don't hit the network. 22 green, analyzer clean.

apps/app_seeds/test/ui/chat_screen_test.dart

@@ -13,6 +13,7 @@ void main() {
       (tester) async {
     final social = await SocialService.fromRootSeedHex('00' * 32);
     final settings = SocialSettings(InMemorySecretStore());
+    await settings.setRelayUrls(const []); // offline: don't hit the network
     LocaleSettings.setLocaleSync(AppLocale.en);
 
     await tester.pumpWidget(