Harden Google sign-in when backend auth fails

Sign out of Firebase and clear API and local profile if the GraphQL exchange or token save fails after Firebase Google sign-in, so the user is not left in a half-authenticated state. Rename the exchange helper to exchangeFirebaseIdTokenWithBackend for clarity.

Login: ignore duplicate Google taps while loading and disable the Google button during the flow. Credential Manager: treat GetCredentialInterruptedException as cancellation and rethrow CancellationException so coroutines cancel correctly.

Tests: assert signOut is invoked when the backend exchange fails.
Made-with: Cursor

app/src/main/kotlin/org/db3/airmq/features/login/GoogleSignInManager.kt

@@ -15,6 +15,7 @@ import androidx.credentials.exceptions.NoCredentialException
 import com.google.android.libraries.identity.googleid.GetGoogleIdOption
 import com.google.android.libraries.identity.googleid.GoogleIdTokenCredential
 import com.google.android.libraries.identity.googleid.GoogleIdTokenParsingException
+import kotlin.coroutines.cancellation.CancellationException
 import org.db3.airmq.R
 
 sealed interface GoogleSignInResult {
@@ -51,12 +52,17 @@ suspend fun launchGoogleSignIn(context: Context): GoogleSignInResult {
     } catch (error: GetCredentialCancellationException) {
         logGoogleSignInError(error)
         GoogleSignInResult.Cancelled
+    } catch (error: GetCredentialInterruptedException) {
+        logGoogleSignInError(error)
+        GoogleSignInResult.Cancelled
     } catch (error: GetCredentialException) {
         logGoogleSignInError(error)
         GoogleSignInResult.Error(context.getString(R.string.toast_oauth_failed))
     } catch (error: GoogleIdTokenParsingException) {
         Log.e(GOOGLE_SIGN_IN_TAG, "Google ID token parsing failed", error)
         GoogleSignInResult.Error(context.getString(R.string.toast_oauth_failed))
+    } catch (e: CancellationException) {
+        throw e
     } catch (error: Throwable) {
         GoogleSignInResult.Error(error.message ?: context.getString(R.string.toast_oauth_failed))
     }

app/src/main/kotlin/org/db3/airmq/features/login/LoginScreen.kt

@@ -213,6 +213,7 @@ private fun LoginScreenContent(
                     containerColor = Color.White,
                     contentColor = Color(0xFF202124),
                     modifier = Modifier.fillMaxWidth(),
+                    enabled = !uiState.isLoading,
                     onClick = { onEvent(Event.GoogleClicked) }
                 )
 

app/src/main/kotlin/org/db3/airmq/features/login/LoginViewModel.kt

@@ -35,6 +35,7 @@ class LoginViewModel @Inject constructor(
     fun onEvent(event: Event) {
         when (event) {
             Event.GoogleClicked -> {
+                if (_uiState.value.isLoading) return
                 _uiState.value = _uiState.value.copy(isLoading = true)
                 _actions.tryEmit(Action.LaunchGoogleSignIn)
             }