[Shortcut Guide] Add page-local search (#49639)

## Summary of the Pull Request

Adds an accessible search box to the Shortcut Guide title bar that
filters shortcuts on the currently selected application page.

The query matches shortcut names, descriptions, modifier names, and
displayed key labels while preserving the existing pinned, recommended,
category, and taskbar grouping.

## PR Checklist

- [x] Closes: #48791
- [x] **Communication:** The UX and behavior were discussed before
implementation
- [x] **Tests:** Added/updated and all pass
- [x] **Localization:** All end-user-facing strings can be localized
- [x] **Dev docs:** Added/updated
- [ ] **New binaries:** Not applicable
- [ ] **Documentation updated:** Not applicable

## Detailed Description of the Pull Request / Additional comments

- Adds a localized title-bar `AutoSuggestBox` with a find icon and UI
Automation identity.
- Filters only the selected app page using case-insensitive matching
across names, descriptions, modifiers, virtual-key display names, and
rendered special-key aliases.
- Keeps only sections containing matches and shows a polite live-region
no-results state with correct pane spacing.
- Preserves the query when switching app pages, but clears it when
Shortcut Guide closes.
- Adds `Ctrl+F` to focus search; the first `Escape` clears a query and
the next closes the overlay.
- Keeps query text local to the UI with no logging or telemetry.

Related issues: #48860 requests several broader navigation/readability
changes; #49459 requests direct physical-key interception rather than
text search.

## Screenshots

### Filter Windows shortcuts by displayed key label

<img
src="https://raw.githubusercontent.com/niels9001/PowerToys/pr-assets-shortcut-guide-search/.github/pr-assets/shortcut-guide-search/windows-alt-filter.png"
width="667" alt="Shortcut Guide Windows page filtered by Alt" />

### Keep the query while switching to the PowerToys page

<img
src="https://raw.githubusercontent.com/niels9001/PowerToys/pr-assets-shortcut-guide-search/.github/pr-assets/shortcut-guide-search/powertoys-opa-filter.png"
width="660" alt="Shortcut Guide PowerToys page filtered by opa" />

## Validation Steps Performed

- Built `ShortcutGuide.Ui` for ARM64 Debug with the repository build
scripts.
- Built `ShortcutGuide.UnitTests` for ARM64 Debug and passed all 23
tests (16 search cases plus 7 existing tests) with `vstest.console.exe`.
- Verified via UIA and guarded keyboard input that name/key-label
filtering updates immediately, empty sections disappear, and no matches
show the localized live-region state.
- Verified the query persists when switching Windows to PowerToys,
`Ctrl+F` focuses search, first `Escape` clears, second `Escape` closes,
and reopening starts with an empty query.
- Rebuilt after the final no-results accessibility and 16px top-margin
adjustment.

---------

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: b8ffa76b-3cf0-4a67-9adb-a13c5dd9f125
Copilot-Session: 4a96c2c2-6954-4784-8257-e0de0fac15a7
Copilot-Session: 1f00def4-e790-4071-96c6-a81c9c2adba5
Copilot-Session: 76e284a6-9a03-4105-bae6-4ed7fc92042d

doc/devdocs/modules/shortcut_guide.md

@@ -25,6 +25,9 @@ The **Hold Windows key** setting is independent of the activation shortcut:
 - **Show taskbar indicators** is the default and always hides the indicators when the Windows key is released.
 - **Open Shortcut Guide** can close on Windows-key release or remain open.
 
+- Use the title-bar search box to filter shortcuts on the selected application page
+- Press Ctrl+F to focus search. Escape clears an active search before dismissing the overlay
+- 
 The hold duration accepts values from 100 through 5,000 milliseconds and defaults to 900 milliseconds.
 
 ## Build and Debug Instructions