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>

desktopApp/build.gradle.kts

@@ -49,6 +49,9 @@ dependencies {
     // Collections
     implementation(libs.kotlinx.collections.immutable)
 
+    // QR code generation (ZXing core)
+    implementation(libs.zxing)
+
     // Testing
     testImplementation(libs.kotlin.test)
     testImplementation(libs.kotlinx.coroutines.test)