CmdPal: Replace FiltersDropDown ComboBox with searchable dropdown (#45747)

## Summary of the Pull Request

Replaces the ComboBox-based filter control with a DropDownButton and
Flyout containing a searchable TextBox and ListView.

- Add type-to-search: typing while button is focused opens the flyout
and filters items by name
- Designed to match appearance of the context menu
- Add keyboard navigation: `Up`/`Down `moves selection from search box,
`Enter` confirms, `Escape` clears search text (or closes if empty), `F4`
opens the dropdown
- Add `Alt+F` shortcut on ShellPage to toggle filter focus
- Style flyout to match ContextMenu (item padding, separators, search
box appearance)
- Show "No results" empty state when search matches nothing
- After confirming selection, return focus to the main search box
- Add accessibility
- Update `FilterTemplateSelector` to support both ComboBoxItem and
ListViewItem containers
- Guard against infinite loop in navigation when only separators exist

## Pictures? Moving!



https://github.com/user-attachments/assets/60e232ae-8cee-4759-a9a7-d7edbf78719e

<img width="315" height="212" alt="image"
src="https://github.com/user-attachments/assets/b6e1a895-064c-47e1-9184-26dbb46fdf05"
/>


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## PR Checklist

- [x] Closes: #41648
<!-- - [ ] Closes: #yyy (add separate lines for additional resolved
issues) -->
- [ ] **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

src/modules/cmdpal/Microsoft.CmdPal.UI/Helpers/KeyModifiers.cs

@@ -0,0 +1,50 @@
+// Copyright (c) Microsoft Corporation
+// The Microsoft Corporation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using Microsoft.UI.Input;
+using Windows.System;
+using Windows.UI.Core;
+
+namespace Microsoft.CmdPal.UI.Helpers;
+
+/// <summary>
+/// Snapshot of the current keyboard modifier state (Ctrl, Alt, Shift, Win).
+/// </summary>
+internal readonly struct KeyModifiers
+{
+    public bool Ctrl { get; }
+
+    public bool Alt { get; }
+
+    public bool Shift { get; }
+
+    public bool Win { get; }
+
+    private KeyModifiers(bool ctrl, bool alt, bool shift, bool win)
+    {
+        Ctrl = ctrl;
+        Alt = alt;
+        Shift = shift;
+        Win = win;
+    }
+
+    /// <summary>
+    /// Gets a snapshot of the modifier keys currently held down.
+    /// </summary>
+    public static KeyModifiers GetCurrent()
+    {
+        var ctrl = InputKeyboardSource.GetKeyStateForCurrentThread(VirtualKey.Control).HasFlag(CoreVirtualKeyStates.Down);
+        var alt = InputKeyboardSource.GetKeyStateForCurrentThread(VirtualKey.Menu).HasFlag(CoreVirtualKeyStates.Down);
+        var shift = InputKeyboardSource.GetKeyStateForCurrentThread(VirtualKey.Shift).HasFlag(CoreVirtualKeyStates.Down);
+        var win = InputKeyboardSource.GetKeyStateForCurrentThread(VirtualKey.LeftWindows).HasFlag(CoreVirtualKeyStates.Down) ||
+                  InputKeyboardSource.GetKeyStateForCurrentThread(VirtualKey.RightWindows).HasFlag(CoreVirtualKeyStates.Down);
+        return new KeyModifiers(ctrl, alt, shift, win);
+    }
+
+    public bool OnlyAlt => Alt && !Ctrl && !Shift && !Win;
+
+    public bool OnlyCtrl => Ctrl && !Alt && !Shift && !Win;
+
+    public bool None => !Ctrl && !Alt && !Shift && !Win;
+}