fix: normalize path separators in scan_dir_recursive for Windows (#3430)

On Windows, Path::strip_prefix produces backslash-separated relative
paths. The update-check matching logic uses rsplit('/') to extract the
install name, so subdirectory skills (e.g. skills/my-skill) never
matched and updates were silently skipped. Replace backslashes with
forward slashes when building the directory string.

src-tauri/src/services/skill.rs

@@ -1964,7 +1964,7 @@ impl SkillService {
                     .strip_prefix(base_dir)
                     .unwrap_or(current_dir)
                     .to_string_lossy()
-                    .to_string()
+                    .replace('\\', "/")
             };
 
             let doc_path = skill_md