Add settings page (#46)

Fixes #37, fixes #47.

Reviewed-on: Nexus/nexus#46

lib/models/settings.dart

@@ -0,0 +1,16 @@
+import "package:flutter/material.dart";
+import "package:freezed_annotation/freezed_annotation.dart";
+part "settings.freezed.dart";
+part "settings.g.dart";
+
+@freezed
+abstract class Settings with _$Settings {
+  const factory Settings({
+    @Default(ThemeMode.system) ThemeMode theme,
+    @Default(true) bool useDynamicTheming,
+    @Default(false) bool linuxMobileMode,
+  }) = _Settings;
+
+  factory Settings.fromJson(Map<String, Object?> json) =>
+      _$SettingsFromJson(json);
+}