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/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();