CmdPal: Bind FilterDropDown selection to the current filter and ensure notifications are raised on UI thread (#41808)

## Summary of the Pull Request

This PR declaratively binds FilterDropDown.SelectedValue to
CurrentFilterId (one-way only; updates in the opposite direction are
handled within the drop-down’s code). It also removes observable
properties and reverts to the UpdateProperty style to ensure property
change notifications are raised on the UI thread, aligning the handling
style with other classes.

## Impact
- Fixed a crash that could occur on pages with filters
- The filter drop-down now correctly syncs with the initially selected
filter when loading a page

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## PR Checklist

- [x] Closes: #41578
- [x] Closes: #41649
- [ ] **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
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for new binaries and localization folder
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pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
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- [ ] **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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## Detailed Description of the Pull Request / Additional comments

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src/modules/cmdpal/Microsoft.CmdPal.UI/Controls/FiltersDropDown.xaml

@@ -74,6 +74,8 @@
         ItemsSource="{x:Bind ViewModel.Filters, Mode=OneWay}"
         PlaceholderText="Filters"
         PreviewKeyDown="FiltersComboBox_PreviewKeyDown"
+        SelectedValue="{x:Bind ViewModel.CurrentFilterId, Mode=OneWay}"
+        SelectedValuePath="Id"
         SelectionChanged="FiltersComboBox_SelectionChanged"
         Style="{StaticResource ComboBoxStyle}"
         Visibility="{x:Bind ViewModel.ShouldShowFilters, Mode=OneWay, Converter={StaticResource BoolToVisibilityConverter}}">