[Settings] Fix FancyZones page scroll bounce at Excluded apps

[niels9001]

Summary

Fixes #25353 — the FancyZones Settings page bounces / jumps when the user scrolls down to the bottom (the Excluded apps expander). Several closed dupes report the same behavior: #31500, #34259, #30164, #45173, #32099, #32361.

Root cause

The Excluded-apps TextBox (lines 283-305 of FancyZonesPage.xaml) forced its own inner ScrollViewer to be permanently enabled with a visible scrollbar, nested inside the page-level ScrollViewer provided by SettingsPageControl:

ScrollViewer.IsVerticalRailEnabled="True"
ScrollViewer.VerticalScrollBarVisibility="Visible"
ScrollViewer.VerticalScrollMode="Enabled"

The expander is IsExpanded="True" and sits at the very bottom of the page, so when you wheel-scroll down the page the pointer ends up over the TextBox. The TextBox''s inner scroller intercepts the wheel events; when it has nothing left to scroll the event bubbles back to the outer page scroller, which tries to over-scroll past content end. The result is the bounce/jump visible in the videos on the issue.

The unbounded MinHeight="160" (with no MaxHeight) made it worse, because the box can grow as users type, triggering layout re-measures during scrolling.

Fix

Minimal change to the same TextBox:

• Drop ScrollViewer.IsVerticalRailEnabled and ScrollViewer.VerticalScrollMode — AcceptsReturn="True" already wires up the inner scroller correctly.
• Change ScrollViewer.VerticalScrollBarVisibility from Visible to Auto, so the inner scroller only competes with the outer one when there is actual overflow to scroll.
• Add MaxHeight="320" so the box cannot keep growing as the user types.

Validation

• Builds clean (tools\build\build.cmd from src\settings-ui\Settings.UI, arm64 Debug, exit 0, 0 errors).
• The same nested-scroller pattern exists on 9 other Settings pages (AlwaysOnTop, GrabAndMove, MouseUtils, MouseWithoutBorders, PowerAccent, PowerLauncher, ShortcutGuide). Keeping this PR atomic to the page called out in The FancyZones settings page bounces when scrolling to the bottom #25353; happy to follow up with a sibling PR for those if reviewers want them in the same pass.

[niels9001]

Tested with a Logi mouse and can confirm that the stuttering is no longer there.