fix(ui): update favicon dynamically based on app theme

Add icon-light.svg and icon-dark.svg with fixed fills (black/white)
instead of relying on CSS prefers-color-scheme media query.

Update WhitelabelingProvider to use useTheme() and switch favicon
based on resolvedTheme (dark/light/unknown).

When custom faviconUrl is set, it takes precedence.

Closes #4100.