fix: normalize path case for style matching on case-insensitive systems
On Windows, when the working directory path case differs from the actual
filesystem path (e.g. `d:\dev` vs `D:\dev`), styles are missing because
Astro treats them as different files.
This normalizes the resolved root path using `fs.realpathSync` to ensure
consistent casing on case-insensitive file systems.
Fixes withastro#14013