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/lib/services/social_settings.dart

@@ -1,12 +1,21 @@
 import '../security/secret_store.dart';
 
+/// A suggested community server the user can join with one tap.
+class RelayPreset {
+  const RelayPreset(this.name, this.url);
+  final String name;
+  final String url;
+}
+
 /// User choices for the (Block 2) social layer: the coarse area to publish/
 /// browse offers in, and which community relays to talk to. Backed by the OS
 /// keystore (via [SecretStore]) to honour "no plaintext at rest" — no
 /// shared_preferences.
 ///
-/// Both default to "unset": empty area and no relays mean the social layer is
-/// simply unavailable, and the app stays fully usable offline.
+/// Relays default to a small set of well-known public servers so the market
+/// works out of the box; the exposure is minimal (offers are opt-in and carry
+/// only a coarse geohash) and the user can swap them for a community server.
+/// The area stays unset until the user picks one (it's inherently personal).
 class SocialSettings {
   SocialSettings(this._store);
 
@@ -15,6 +24,17 @@ class SocialSettings {
   static const _areaKey = 'tane.social.area_geohash';
   static const _relaysKey = 'tane.social.relays';
 
+  /// Suggested servers shown as one-tap options in the sharing setup.
+  static const List<RelayPreset> presets = [
+    RelayPreset('nos.lol', 'wss://nos.lol'),
+    RelayPreset('Damus', 'wss://relay.damus.io'),
+    RelayPreset('Primal', 'wss://relay.primal.net'),
+  ];
+
+  /// Applied automatically for new users (a subset of [presets]) so sharing
+  /// works from the first launch, until the user changes it.
+  static const List<String> defaultRelays = ['wss://nos.lol', 'wss://relay.damus.io'];
+
   /// The user's coarse area as a low-precision geohash (may be empty). Set
   /// manually or from device location; the transport coarsens it further.
   Future<String> areaGeohash() async => (await _store.read(_areaKey)) ?? '';
@@ -22,11 +42,12 @@ class SocialSettings {
   Future<void> setAreaGeohash(String geohash) =>
       _store.write(_areaKey, geohash.trim().toLowerCase());
 
-  /// Configured community relay URLs (empty by default — the user adds their
-  /// own; we bundle none, to avoid leaking metadata to third-party relays).
+  /// The relays to talk to. When the user has never configured any, falls back
+  /// to [defaultRelays]. Once they save a choice (even an empty one), that wins
+  /// — so a privacy-minded user can turn the network off entirely.
   Future<List<String>> relayUrls() async {
     final raw = await _store.read(_relaysKey);
-    if (raw == null || raw.isEmpty) return const [];
+    if (raw == null) return const [...defaultRelays]; // never configured
     return raw.split('\n').where((s) => s.trim().isNotEmpty).toList();
   }