feat: Add settings to control play button visibility on home shelves

This commit introduces new functionality to customize the visibility of the
play button on home page shelves.

Key changes:

- Added `HomePageSettings` to `AppSettings` to store your preferences:
    - `showPlayButtonOnContinueShelves`: Controls visibility on "Continue Listening" and "Continue Series" shelves (default: true).
    - `showPlayButtonOnAllShelves`: Controls visibility on other shelves (default: false).
- Modified `BookHomeShelf` to respect these settings when rendering books.
- Created a new "Home Page Settings" page under "Appearance" in App Settings, allowing you to toggle these two options.
- Added comprehensive unit and widget tests to cover the new settings model, the conditional logic in `BookHomeShelf`, and the functionality of the new settings page.

test/settings/app_settings_test.dart

@@ -0,0 +1,51 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:vaani/settings/models/app_settings.dart';
+
+void main() {
+  group('AppSettings', () {
+    test('initializes with default HomePageSettings', () {
+      const appSettings = AppSettings();
+      expect(appSettings.homePageSettings.showPlayButtonOnContinueShelves, isTrue);
+      expect(appSettings.homePageSettings.showPlayButtonOnAllShelves, isFalse);
+    });
+
+    test('HomePageSettings can be updated', () {
+      const initialSettings = AppSettings();
+      final updatedSettings = initialSettings.copyWith(
+        homePageSettings: initialSettings.homePageSettings.copyWith(
+          showPlayButtonOnContinueShelves: false,
+          showPlayButtonOnAllShelves: true,
+        ),
+      );
+
+      expect(updatedSettings.homePageSettings.showPlayButtonOnContinueShelves, isFalse);
+      expect(updatedSettings.homePageSettings.showPlayButtonOnAllShelves, isTrue);
+    });
+
+    test('HomePageSettings are correctly serialized and deserialized', () {
+      const originalSettings = AppSettings(
+        homePageSettings: HomePageSettings(
+          showPlayButtonOnContinueShelves: false,
+          showPlayButtonOnAllShelves: true,
+        ),
+      );
+
+      final json = originalSettings.toJson();
+      final deserializedSettings = AppSettings.fromJson(json);
+
+      expect(deserializedSettings.homePageSettings.showPlayButtonOnContinueShelves, isFalse);
+      expect(deserializedSettings.homePageSettings.showPlayButtonOnAllShelves, isTrue);
+      expect(deserializedSettings, originalSettings);
+    });
+
+    test('Default AppSettings serialization and deserialization', () {
+      const originalSettings = AppSettings();
+      final json = originalSettings.toJson();
+      final deserializedSettings = AppSettings.fromJson(json);
+
+      expect(deserializedSettings.homePageSettings.showPlayButtonOnContinueShelves, isTrue);
+      expect(deserializedSettings.homePageSettings.showPlayButtonOnAllShelves, isFalse);
+      expect(deserializedSettings, originalSettings);
+    });
+  });
+}