feat(android): "Change server URL" link on Login (parity map: Auth)

Ports flutter_client/lib/auth/login_screen.dart:86-89 — a TextButton
under the Sign-in button that routes to the ServerUrl screen, so a
wrong server URL is recoverable pre-login without reinstalling or
signing out from Settings. Disabled while a submit is in flight.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

android/app/src/main/java/com/fabledsword/minstrel/auth/ui/LoginScreen.kt

@@ -14,6 +14,7 @@ import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.OutlinedTextField
 import androidx.compose.material3.Surface
 import androidx.compose.material3.Text
+import androidx.compose.material3.TextButton
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.runtime.getValue
@@ -28,6 +29,7 @@ import androidx.lifecycle.compose.collectAsStateWithLifecycle
 import androidx.navigation.NavHostController
 import com.fabledsword.minstrel.nav.Home
 import com.fabledsword.minstrel.nav.Login
+import com.fabledsword.minstrel.nav.ServerUrl
 
 @Composable
 fun LoginScreen(
@@ -55,6 +57,7 @@ fun LoginScreen(
             onUsernameChange = viewModel::setUsername,
             onPasswordChange = viewModel::setPassword,
             onSubmit = viewModel::submit,
+            onChangeServerUrl = { navController.navigate(ServerUrl) },
         )
     }
 }
@@ -65,6 +68,7 @@ private fun LoginForm(
     onUsernameChange: (String) -> Unit,
     onPasswordChange: (String) -> Unit,
     onSubmit: () -> Unit,
+    onChangeServerUrl: () -> Unit,
 ) {
     Column(
         modifier = Modifier
@@ -112,6 +116,12 @@ private fun LoginForm(
             )
         }
         SubmitButton(state = state, onSubmit = onSubmit)
+        TextButton(
+            onClick = onChangeServerUrl,
+            enabled = !state.isSubmitting,
+        ) {
+            Text("Change server URL")
+        }
     }
 }