Fix window caption buttons invisible in Light theme

[StuartMeeks]

Problem

The minimise / maximise / close glyphs were always the default white, so they were barely visible against the light title bar. The app sets ExtendsContentIntoTitleBar + a custom title bar but never configured AppWindow.TitleBar's button colours, so they fell back to white regardless of theme.

Fix

• MainWindow.ApplyTheme is now the single theming entry point (made public). It sets the content RequestedTheme and the AppWindow.TitleBar caption-button colours, derived from the content's resolved ActualTheme — so the glyphs always contrast with the active theme. Button backgrounds stay transparent so the Mica backdrop still shows through.
• WindowsThemeApplier now delegates to MainWindow.ApplyTheme instead of duplicating the theme switch, so a runtime theme change from Settings updates the caption buttons too.
• An ActualThemeChanged handler refreshes the glyphs when the OS flips light/dark while the preference is System.

Behaviour

• Light → near-black glyphs, clearly visible.
• Dark → white (unchanged).
• System → follows the OS, live on OS theme flip.
• Switching theme in Settings updates the buttons immediately (no restart).

Testing

• 226 Core unit tests pass.
• WinUI head compile-verified on the Windows build agent: 0 warnings, 0 errors.
• Visual check (light/dark/system) is for the reviewer, since caption rendering can't be verified headless.

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