feat: add store registry URL management, port iOS handlers, and clean up store UI

Add set/get/clear store registry URL method channel handlers on Android,
iOS, and Go backend so users can configure a custom extension repository.

Store tab now shows a setup screen when no registry URL is configured,
with a cleaner layout (removed redundant description and helper text)
and visible TextField borders for dark theme.

Minor comment and formatting cleanups across several files.

android/app/src/main/kotlin/com/zarz/spotiflac/MainActivity.kt

@@ -3064,6 +3064,25 @@ class MainActivity: FlutterFragmentActivity() {
                             }
                             result.success(null)
                         }
+                        "setStoreRegistryUrl" -> {
+                            val registryUrl = call.argument<String>("registry_url") ?: ""
+                            withContext(Dispatchers.IO) {
+                                Gobackend.setStoreRegistryURLJSON(registryUrl)
+                            }
+                            result.success(null)
+                        }
+                        "getStoreRegistryUrl" -> {
+                            val response = withContext(Dispatchers.IO) {
+                                Gobackend.getStoreRegistryURLJSON()
+                            }
+                            result.success(response)
+                        }
+                        "clearStoreRegistryUrl" -> {
+                            withContext(Dispatchers.IO) {
+                                Gobackend.clearStoreRegistryURLJSON()
+                            }
+                            result.success(null)
+                        }
                         "getStoreExtensions" -> {
                             val forceRefresh = call.argument<Boolean>("force_refresh") ?: false
                             val response = withContext(Dispatchers.IO) {