feat(extensions): complete signed-session grants from help dialog

Add a platform bridge to finish session grants on Android and iOS with
JSON success validation, let users paste callback URLs from the
clipboard, and auto-dismiss the verification help dialog after grant.

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

@@ -2115,6 +2115,9 @@ class MainActivity: FlutterFragmentActivity() {
                     Gobackend.setExtensionAuthCodeByID(extId, code)
                     Gobackend.invokeExtensionActionJSON(extId, "completeSpotifyLogin")
                 }
+                if (isSessionGrant) {
+                    requireSuccessfulExtensionAction(extId, "completeGrant", json)
+                }
                 android.util.Log.i("SpotiFLAC", "Extension callback complete for $extId: $json")
                 if (isSessionGrant) {
                     withContext(Dispatchers.Main) {
@@ -2132,6 +2135,23 @@ class MainActivity: FlutterFragmentActivity() {
         }
     }
 
+    private fun requireSuccessfulExtensionAction(extensionId: String, actionName: String, response: String) {
+        val obj = try {
+            JSONObject(response)
+        } catch (e: Exception) {
+            throw IllegalStateException(
+                "Extension $actionName for $extensionId returned invalid JSON: ${response.take(240)}"
+            )
+        }
+        if (obj.optBoolean("success", false)) {
+            return
+        }
+        val error = obj.optString("error")
+            .ifBlank { obj.optString("message") }
+            .ifBlank { response.take(240) }
+        throw IllegalStateException("Extension $actionName failed for $extensionId: $error")
+    }
+
     private fun notifySessionGrantCompleted(extensionId: String, success: Boolean) {
         val payload = mapOf(
             "extension_id" to extensionId,
@@ -3340,6 +3360,16 @@ class MainActivity: FlutterFragmentActivity() {
                             }
                             result.success(null)
                         }
+                        "completeExtensionSessionGrant" -> {
+                            val extensionId = call.argument<String>("extension_id") ?: ""
+                            val grant = call.argument<String>("grant") ?: ""
+                            withContext(Dispatchers.IO) {
+                                Gobackend.setExtensionSessionGrantByID(extensionId, grant)
+                                val json = Gobackend.invokeExtensionActionJSON(extensionId, "completeGrant")
+                                requireSuccessfulExtensionAction(extensionId, "completeGrant", json)
+                            }
+                            result.success(true)
+                        }
                         "setExtensionTokens" -> {
                             val extensionId = call.argument<String>("extension_id") ?: ""
                             val accessToken = call.argument<String>("access_token") ?: ""