Add settings page (#46)

Fixes #37, fixes #47.

Reviewed-on: Nexus/nexus#46

lib/controllers/settings_file_controller.dart

@@ -0,0 +1,23 @@
+import "dart:io";
+import "package:flutter_riverpod/flutter_riverpod.dart";
+import "package:path/path.dart";
+import "package:path_provider/path_provider.dart";
+import "package:xdg_directories/xdg_directories.dart";
+
+class SettingsFileController extends AsyncNotifier<File> {
+  @override
+  Future<File> build() async {
+    final directory = await switch (Platform.isLinux) {
+      true => Directory(
+        join(configHome.absolute.path, "nexus"),
+      ).create(recursive: true),
+      false => getApplicationSupportDirectory(),
+    };
+
+    return File(join(directory.absolute.path, "config.json"));
+  }
+
+  static final provider = AsyncNotifierProvider<SettingsFileController, File>(
+    SettingsFileController.new,
+  );
+}