feat(desktop): add nostrconnect:// login flow for QR-based signer pairing

Adds client-initiated NIP-46 (nostrconnect://) flow so users can pair
with a remote signer (e.g. Amber) by scanning a QR code instead of
manually copying a bunker:// URI.

Changes:
- Fix RemoteSignerManager to decrypt NIP-44 content before parsing
- Add CoroutineScope to NostrSignerRemote for async event callbacks
- Add loginWithNostrConnect() to AccountManager with 120s timeout
- Add QrCodeCanvas composable using ZXing for QR generation
- Add "Connect" tab to LoginCard showing QR + copyable URI + waiting state
- Wire nostrconnect flow through LoginScreen

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

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip46RemoteSigner/signer/NostrSignerRemote.kt

@@ -42,6 +42,11 @@ import com.vitorpamplona.quartz.nip46RemoteSigner.NostrConnectEvent
 import com.vitorpamplona.quartz.nip57Zaps.LnZapPrivateEvent
 import com.vitorpamplona.quartz.nip57Zaps.LnZapRequestEvent
 import com.vitorpamplona.quartz.utils.Hex
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.SupervisorJob
+import kotlinx.coroutines.cancel
+import kotlinx.coroutines.launch
 
 class NostrSignerRemote(
     val signer: NostrSignerInternal,
@@ -51,6 +56,8 @@ class NostrSignerRemote(
     val permissions: String? = null,
     val secret: String? = null,
 ) : NostrSigner(signer.pubKey) {
+    private val scope = CoroutineScope(Dispatchers.Default + SupervisorJob())
+
     private val manager =
         RemoteSignerManager(
             signer = signer,
@@ -69,7 +76,13 @@ class NostrSignerRemote(
                 ),
         ) { event ->
             if (event is NostrConnectEvent) {
-                manager.newResponse(event)
+                scope.launch {
+                    try {
+                        manager.newResponse(event)
+                    } catch (_: Exception) {
+                        // Ignore events we can't decrypt or parse
+                    }
+                }
             }
         }
 
@@ -79,6 +92,7 @@ class NostrSignerRemote(
 
     fun closeSubscription() {
         subscription.close()
+        scope.cancel()
     }
 
     override fun isWriteable(): Boolean = true

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip46RemoteSigner/signer/RemoteSignerManager.kt

@@ -41,8 +41,9 @@ class RemoteSignerManager(
 ) {
     private val awaitingRequests = LargeCache<String, Continuation<BunkerResponse>>()
 
-    fun newResponse(responseEvent: NostrConnectEvent) {
-        val bunkerResponse = OptimizedJsonMapper.fromJsonTo<BunkerResponse>(responseEvent.content)
+    suspend fun newResponse(responseEvent: NostrConnectEvent) {
+        val decryptedJson = signer.decrypt(responseEvent.content, remoteKey)
+        val bunkerResponse = OptimizedJsonMapper.fromJsonTo<BunkerResponse>(decryptedJson)
         awaitingRequests.get(bunkerResponse.id)?.resume(bunkerResponse)
     }