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.

apps/app_seeds/lib/ui/profile_screen.dart

@@ -30,6 +30,7 @@ class ProfileScreen extends StatefulWidget {
 class _ProfileScreenState extends State<ProfileScreen> {
   final _name = TextEditingController();
   final _about = TextEditingController();
+  final _g1 = TextEditingController();
   bool _loading = true;
   bool _saving = false;
 
@@ -42,6 +43,7 @@ class _ProfileScreenState extends State<ProfileScreen> {
   Future<void> _load() async {
     _name.text = await widget.profileStore.name();
     _about.text = await widget.profileStore.about();
+    _g1.text = await widget.profileStore.g1();
     if (mounted) setState(() => _loading = false);
   }
 
@@ -57,7 +59,8 @@ class _ProfileScreenState extends State<ProfileScreen> {
     final messenger = ScaffoldMessenger.of(context);
     final t = context.t;
     setState(() => _saving = true);
-    await widget.profileStore.save(name: _name.text, about: _about.text);
+    await widget.profileStore
+        .save(name: _name.text, about: _about.text, g1: _g1.text);
 
     // Best-effort publish to the network so peers see the name.
     final relays = await widget.settings.relayUrls();
@@ -67,6 +70,7 @@ class _ProfileScreenState extends State<ProfileScreen> {
         await session.profile.publish(
           name: _name.text.trim(),
           about: _about.text.trim(),
+          g1: _g1.text.trim(),
         );
         await session.close();
       } catch (_) {
@@ -82,6 +86,7 @@ class _ProfileScreenState extends State<ProfileScreen> {
   void dispose() {
     _name.dispose();
     _about.dispose();
+    _g1.dispose();
     super.dispose();
   }
 
@@ -116,6 +121,16 @@ class _ProfileScreenState extends State<ProfileScreen> {
                     helperText: t.profile.aboutHint,
                   ),
                 ),
+                const SizedBox(height: 16),
+                TextField(
+                  key: const Key('profile.g1'),
+                  controller: _g1,
+                  decoration: InputDecoration(
+                    labelText: t.profile.g1,
+                    helperText: t.profile.g1Hint,
+                    helperMaxLines: 2,
+                  ),
+                ),
                 const SizedBox(height: 24),
                 FilledButton(
                   key: const Key('profile.save'),