feat: add Storage Access Framework (SAF) support for Android 10+

- Add SAF tree picker and persistent URI storage in settings
- Implement SAF file operations: exists, delete, stat, copy, create
- Update download pipeline to support SAF content URIs
- Add fallback to app-private storage when SAF write fails
- Support SAF in library scan with DocumentFile traversal
- Add history item repair for missing SAF URIs
- Create file_access.dart utilities for abstracted file operations
- Update Tidal/Qobuz/Amazon/Extensions for SAF-aware output
- Add runPostProcessingV2 API for SAF content URIs
- Update screens (album, artist, queue, track) for SAF awareness

Resolves Android 10+ scoped storage permission issues

ios/Runner/AppDelegate.swift

@@ -639,6 +639,14 @@ import Gobackend  // Import Go framework
             let response = GobackendRunPostProcessingJSON(filePath, metadataJson, &error)
             if let error = error { throw error }
             return response
+
+        case "runPostProcessingV2":
+            let args = call.arguments as! [String: Any]
+            let inputJson = args["input"] as? String ?? ""
+            let metadataJson = args["metadata"] as? String ?? ""
+            let response = GobackendRunPostProcessingV2JSON(inputJson, metadataJson, &error)
+            if let error = error { throw error }
+            return response
             
         case "getPostProcessingProviders":
             let response = GobackendGetPostProcessingProvidersJSON(&error)