[Quick Accent] Fix missing last-item selection highlight (#49820)

## Summary of the Pull Request

Fixes a pre-existing Quick Accent rendering edge case where the last
character is selected and inserted correctly, but its selection
highlight is missing when a long character list is scrolled to the end.

The horizontal items panel now leaves a symmetric 1-DIP margin outside
its edge containers. This keeps `ScrollIntoView` from landing exactly at
the maximum scroll offset, where WinUI can clip the last item's selected
visual. The symmetric margin preserves short-list centering and works in
both flow directions.

The behavior was reproduced on both the #49633 baseline and #49655, so
it was not introduced by #49655.

## PR Checklist

- [ ] Closes: N/A — discovered while validating #49655; no matching
issue is currently filed
- [ ] **Communication:** Draft PR for maintainer discussion
- [x] **Tests:** Existing tests pass; the XAML compositor edge was also
covered by a deterministic GUI regression run
- [x] **Localization:** N/A — no end-user-facing strings changed
- [x] **Dev docs:** N/A — no developer-facing behavior or contract
changed
- [x] **New binaries:** N/A — no binaries or dependencies added
- [x] **Documentation updated:** N/A — no documentation impact

## Detailed Description of the Pull Request / Additional comments

With the overflowing horizontal list at its maximum offset, the last
`ListViewItem` remains realized, selected, and visible, and its
`SelectionIndicator` has opacity 1. However, WinUI does not paint that
selected visual. Adding one DIP of scroll extent after the edge
container avoids that boundary condition without changing selection or
keyboard-navigation logic.

The panel margin is symmetric so non-overflowing lists remain centered
and RTL layouts receive the same protection at either edge.
Infinite-width measurement already includes the margin, so the existing
window sizing path needs no special case.

## Validation Steps Performed

- Built `PowerAccent.UI` with `Debug|x64` using `tools/build/build.ps1`.
- Built `PowerAccent.Common.UnitTests` and `PowerAccent.Core.UnitTests`
with `Debug|x64`.
- Ran both suites with `vstest.console.exe`: **55/55 passed**.
- Reproduced the original failure by holding `A`, pressing `Space`, then
`Shift+Space` before reveal: the selected `ₐ` was inserted but the
baseline showed no selection-highlight pixels.
- Repeated the same first-frame scenario **20 times** after the fix: all
first and stable frames rendered the highlight (4441 detected accent
pixels), with the foreground window preserved.
- Repeated insertion validation in a dedicated text host **5 times**:
all runs inserted `U+2090` (`ₐ`), rendered the highlight, and preserved
foreground focus.
- Ran `git diff --check` successfully.

Co-authored-by: Yu Leng (from Dev Box) <yuleng@microsoft.com>

src/modules/poweraccent/PowerAccent.UI/PowerAccentXAML/SelectorControl.xaml

@@ -38,7 +38,12 @@
                     </ListView.ItemContainerTransitions>
                     <ListView.ItemsPanel>
                         <ItemsPanelTemplate>
-                            <StackPanel Orientation="Horizontal" />
+                            <!--
+                                Leave one DIP of scroll extent outside the first and last containers.
+                                When ScrollIntoView lands exactly at the maximum offset, WinUI can
+                                clip the last item's selection visual even though the item is visible.
+                            -->
+                            <StackPanel Margin="1,0,1,0" Orientation="Horizontal" />
                         </ItemsPanelTemplate>
                     </ListView.ItemsPanel>
                     <!--