feat(block2): Ğ1 — publish a Ğ1 address + 'Pay in Ğ1' in chat (levels 1-2)

The optional Ğ1 (free currency) integration, per g1-integration.md.
- Profile: an optional 'Ğ1 address' field, persisted locally and published in
  your NIP-01 kind:0 metadata (separate from the Nostr key).
- commons_core: UserProfile/ProfileTransport carry a 'g1' field (round-trip
  tested).
- Chat: when the peer published a Ğ1 address, a 'Pay in Ğ1' app-bar action opens
  their wallet (url_launcher) so value moves in the wallet, not the app — copies
  the address as a fallback. No payment rails in-app (sharing-model §4.3).

NOTE (vjrj): the wallet deep-link uses the Cesium web wallet as a universal
fallback; swap the URI in chat_screen._payG1 for the Ğ1nkgo/Ğecko scheme you
prefer. Analyzer clean (both packages); profile transport test covers g1.

packages/commons_core/lib/src/social/nostr/nostr_profile_transport.dart

@@ -19,11 +19,13 @@ class NostrProfileTransport implements ProfileTransport {
     String name = '',
     String about = '',
     String picture = '',
+    String g1 = '',
   }) async {
     final content = jsonEncode({
       if (name.isNotEmpty) 'name': name,
       if (about.isNotEmpty) 'about': about,
       if (picture.isNotEmpty) 'picture': picture,
+      if (g1.isNotEmpty) 'g1': g1,
     });
     final event = Event.from(
       kind: kindMetadata,
@@ -50,6 +52,7 @@ class NostrProfileTransport implements ProfileTransport {
         name: (map['name'] as String?) ?? '',
         about: (map['about'] as String?) ?? '',
         picture: (map['picture'] as String?) ?? '',
+        g1: (map['g1'] as String?) ?? '',
       );
     } catch (_) {
       return UserProfile(pubkeyHex: pubkeyHex);

packages/commons_core/lib/src/social/profile_transport.dart

@@ -6,6 +6,7 @@ class UserProfile {
     this.name = '',
     this.about = '',
     this.picture = '',
+    this.g1 = '',
   });
 
   final String pubkeyHex;
@@ -13,7 +14,12 @@ class UserProfile {
   final String about;
   final String picture;
 
-  bool get isEmpty => name.isEmpty && about.isEmpty && picture.isEmpty;
+  /// Optional Ğ1 (Duniter) address the person chose to publish, so peers can pay
+  /// them in Ğ1. Separate from the Nostr key; empty when not shared.
+  final String g1;
+
+  bool get isEmpty =>
+      name.isEmpty && about.isEmpty && picture.isEmpty && g1.isEmpty;
 }
 
 /// Publish/fetch public profile metadata. Kind:0 is replaceable — publishing
@@ -21,7 +27,7 @@ class UserProfile {
 /// sibling of the offer/message/trust transports over the same connection.
 abstract interface class ProfileTransport {
   /// Publishes (replaces) this identity's profile.
-  Future<void> publish({String name, String about, String picture});
+  Future<void> publish({String name, String about, String picture, String g1});
 
   /// Fetches [pubkeyHex]'s latest profile, or null if they never published one.
   Future<UserProfile?> fetch(String pubkeyHex);

packages/commons_core/test/social/nostr_profile_test.dart

@@ -16,8 +16,11 @@ void main() {
   test('publish then fetch round-trips a profile (kind:0)', () async {
     final alice = await idFor(1);
     final aliceConn = await NostrConnection.connect(relay.url, identity: alice);
-    await NostrProfileTransport(aliceConn)
-        .publish(name: 'Alicia', about: 'Guardo tomate rosa');
+    await NostrProfileTransport(aliceConn).publish(
+      name: 'Alicia',
+      about: 'Guardo tomate rosa',
+      g1: 'G1abc123',
+    );
 
     // Anyone else can fetch it.
     final bob = await idFor(2);
@@ -28,6 +31,7 @@ void main() {
     expect(fetched, isNotNull);
     expect(fetched!.name, 'Alicia');
     expect(fetched.about, 'Guardo tomate rosa');
+    expect(fetched.g1, 'G1abc123');
 
     await aliceConn.close();
     await bobConn.close();