fixed light switch shortcut not working issue (#42340)

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fix/what does it change/how was it tested (even manually, if necessary)?
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## Summary of the Pull Request
This pull request refactors how the `ToggleThemeHotkey` property is
handled in the LightSwitch settings and view model. The changes simplify
property management by directly referencing the property within
`LightSwitchProperties` and ensure that the hotkey setting is
consistently updated and serialized.

**Settings property management:**

* In `LightSwitchSettings.Clone()`, the `LightSwitchProperties` object
now directly references the existing property instances instead of
creating new ones, and also includes the `ToggleThemeHotkey` property.

**View model property handling:**

* In `LightSwitchViewModel.ToggleThemeActivationShortcut`, the getter
and setter now directly access and update
`ModuleSettings.Properties.ToggleThemeHotkey.Value`, removing the need
for a backing field and ensuring changes are properly serialized and
notified.
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## PR Checklist

- [x] Closes: #42330
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

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or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

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wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

---------

Signed-off-by: Shawn Yuan (from Dev Box) <shuaiyuan@microsoft.com>

src/settings-ui/Settings.UI/SettingsXAML/Views/LightSwitchPage.xaml.cs

@@ -73,6 +73,7 @@ namespace Microsoft.PowerToys.Settings.UI.Views
 
             this.InitializeComponent();
             this.Loaded += LightSwitchPage_Loaded;
+            this.Loaded += (s, e) => ViewModel.OnPageLoaded();
         }
 
         private void LightSwitchPage_Loaded(object sender, RoutedEventArgs e)