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/services/social_settings_test.dart

@@ -15,12 +15,18 @@ void main() {
   late SocialSettings settings;
   setUp(() => settings = SocialSettings(FakeSecretStore()));
 
-  test('defaults: empty area, no relays, not configured', () async {
+  test('defaults: empty area, but relays fall back to the defaults', () async {
     expect(await settings.areaGeohash(), '');
-    expect(await settings.relayUrls(), isEmpty);
+    expect(await settings.relayUrls(), SocialSettings.defaultRelays);
+    // Not configured yet: the area is still unset.
     expect(await settings.isConfigured, isFalse);
   });
 
+  test('an explicitly empty relay choice turns the network off', () async {
+    await settings.setRelayUrls(const []);
+    expect(await settings.relayUrls(), isEmpty);
+  });
+
   test('stores and normalises the area geohash (trim + lowercase)', () async {
     await settings.setAreaGeohash('  SP3E9  ');
     expect(await settings.areaGeohash(), 'sp3e9');
@@ -36,10 +42,9 @@ void main() {
         ['wss://relay.comunes.org', 'wss://seeds.example.org']);
   });
 
-  test('is configured only with both an area and at least one relay', () async {
+  test('is configured once an area is set (relays are defaulted)', () async {
+    expect(await settings.isConfigured, isFalse); // no area yet
     await settings.setAreaGeohash('sp3e9');
-    expect(await settings.isConfigured, isFalse);
-    await settings.setRelayUrls(['wss://relay.comunes.org']);
     expect(await settings.isConfigured, isTrue);
   });
 }

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(

apps/app_seeds/test/ui/market_screen_test.dart

@@ -110,6 +110,7 @@ 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(
@@ -136,6 +137,7 @@ 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(