fix(controls): Fix LeftFluent nav item hover invisible in Light theme

[ACbye]

Pull request type

Please check the type of change your PR introduces:

• Update
• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes

What is the current behavior?

LeftFluent NavigationViewItem hover has no visible feedback in Light theme.
The IsMouseOver trigger and the selected brush both reference ControlFillColorDefault,
which resolves to #B3FFFFFF (70% white) in Light theme. Overlayed on the #FAFAFA
application background, the result is #FEFEFE — a delta of only +4, imperceptible.

Additionally, SubtleFillColorSecondary is #09000000 (3.5% black), causing all
SubtleFill-dependent controls to have very weak hover feedback in Light theme,
while Dark theme achieves +13 with the same token name.

Fixes #1729

What is the new behavior?

• NavigationViewItemBackgroundSelectedLeftFluent references SubtleFillColorSecondary instead of ControlFillColorDefault
• SubtleFillColorSecondary opacity increased from 3.5% (#09000000) to 7.84% (#14000000) for perceptual parity with Dark theme's +13

Theme	Before	After	Delta
Light hover	#FEFEFE (Δ+4)	#E6E6E6 (Δ−20)	visible
Dark hover	#2D2D2D (Δ+13)	unchanged	unchanged

Other information

Only Light.xaml (2 lines) changed. Dark theme and Left/LeftMinimal modes are unaffected.