ui: collapse SMS fallback fields when disabled (matching email)

SMS fields (number / quota / carrier / balance) are now hidden when the
toggle is off. Previously entered values are preserved, just hidden.
Email forwarding already had this pattern. Settings page stays compact.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/src/main/java/com/a2i/forwarder/ui/screens/SettingsScreen.kt

@@ -260,6 +260,7 @@ fun SettingsScreen(onOpenLog: () -> Unit) {
                     checked = smsFallback,
                 ) { v -> app.appScope.launch { app.settings.setSmsFallback(v) } }
 
+                if (smsFallback) {
                     Spacer(Modifier.size(8.dp))
                     OutlinedTextField(
                         value = smsTargetInput,
@@ -328,6 +329,7 @@ fun SettingsScreen(onOpenLog: () -> Unit) {
                     )
                 }
             }
+        }
 
         item {
             SectionCard("应用图标", subtitle = "导出 App 图标后，Bark 通知可按包名显示图标") {