Improve tvOS settings UI styling and navigation

- Add TVOSPlainToggleStyle for cleaner toggle appearance on tvOS
- Remove focus overlays from settings navigation links and buttons
- Apply plain button and list styles across all settings screens
- Implement custom system controls picker for tvOS to avoid focus overlay
- Update SettingsPickerModifier with platform-specific styling

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Shared/Settings/TVOSPlainToggleStyle.swift

@@ -0,0 +1,17 @@
+import SwiftUI
+
+#if os(tvOS)
+struct TVOSPlainToggleStyle: ToggleStyle {
+    func makeBody(configuration: Configuration) -> some View {
+        Button(action: { configuration.isOn.toggle() }) {
+            HStack {
+                configuration.label
+                Spacer()
+                Image(systemName: configuration.isOn ? "checkmark.circle.fill" : "circle")
+                    .foregroundColor(configuration.isOn ? .accentColor : .secondary)
+            }
+        }
+        .buttonStyle(.plain)
+    }
+}
+#endif