fix: include relay hint in naddr for app events

All encodeShareableIdentifier calls passed empty relay lists,
causing other Nostr clients to fail resolving NIP-82 app events
since they don't know to query relay.zapstore.dev.

Adds kDefaultRelay constant with invariant comment and uses it
for app naddr encoding (app_card, app_detail_screen). Stack naddr
calls (user_screen, app_stack_container) keep empty relay lists
since stacks live on social relays, not the app catalog relay.

Closes #343

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/constants/app_constants.dart

@@ -39,6 +39,12 @@ bool appStackEventFilter(Map<String, dynamic> event) {
   return false;
 }
 
+/// Authoritative relay for NIP-82 software application events (kind 32267).
+/// Used as the relay hint in naddr encoding so other clients can resolve app events.
+/// Invariant: all Zapstore-published app events are available on this relay.
+/// Stack events (social relays) should NOT use this hint.
+const kDefaultRelay = 'wss://relay.zapstore.dev';
+
 /// Amber signer package ID
 const kAmberPackageId = 'com.greenart7c3.nostrsigner';
 

lib/screens/app_detail_screen.dart

@@ -633,7 +633,7 @@ class _AppDetailContent extends HookConsumerWidget {
         identifier: app.identifier,
         author: app.pubkey,
         kind: app.event.kind,
-        relays: [],
+        relays: const [kDefaultRelay],
       ),
     );
     return 'https://zapstore.dev/apps/$naddr';

lib/widgets/app_card.dart

@@ -66,7 +66,7 @@ class AppCard extends HookConsumerWidget {
             identifier: app!.identifier,
             author: app!.pubkey,
             kind: app!.event.kind,
-            relays: const [],
+            relays: const [kDefaultRelay],
           ),
         );
         context.push('/$first/app/$naddr');