Fix low color contrast of spatial audio caption in dark mode

[HariniMalothu17]

Summary

On the Sound sample page, the caption "Can only enable spatial audio when sound is on!" under the Enable Spatial Audio checkbox has insufficient color contrast in dark mode.

It used:

<TextBlock
    FontStyle="Italic"
    Foreground="{ThemeResource SystemColorHotlightColor}"
    Style="{ThemeResource CaptionTextBlockStyle}"
    Text="Can only enable spatial audio when sound is on!" />

SystemColorHotlightColor is the hyperlink text color (it is even described that way in the gallery's own Samples/Color/brushes.json). It is not part of the theme's text-color ramp, so in dark mode the caption renders at roughly 2.78:1 against the page background - below the 4.5:1 minimum (MAS 1.4.3 - Contrast (Minimum)).

Fix

Use the theme-aware TextFillColorSecondaryBrush, which adapts to light and dark themes and meets the contrast requirement, for this secondary caption text:

Foreground="{ThemeResource TextFillColorSecondaryBrush}"

One-line change in SoundPage.xaml. No behavioral change.

Repro (before fix)

1. Set Settings -> App theme -> Dark.
2. Open the Sound page.
3. Observe the italic caption under the Enable Spatial Audio checkbox - it is a faint hyperlink-blue that fails 4.5:1 contrast.

After the fix, the caption uses a theme-aware secondary text color that passes the contrast requirement in both themes.

Testing

• Built WinUIGallery (Debug/x64) -> 0 errors.
• Verified the caption uses a contrast-compliant, theme-aware brush.
• Aligns with the gallery's accessibility conventions (enforced by the Axe.Windows UI test scans).

[HariniMalothu17]

/azp run