refactor: idiomatic Rust cleanup and quality improvements

- Replace .map().unwrap_or(false) with .is_some_and()/.is_ok_and()
- Use path.display() instead of {:?} for user-facing messages
- Replace Option<Option<Vec<String>>> with GamescopeUpdate enum
- Replace manual parent-walking loops with .ancestors() iterators
- Simplify kill()/kill_all() signatures to return () instead of Result
- Use tokio::task::spawn_blocking instead of hand-rolled thread+oneshot
- Read /proc/self/status for UID instead of spawning id subprocess
- Build Exec= line directly in render_desktop instead of string-replace
- Bump PKGBUILD pkgrel to 6

Cargo.lock

@@ -4439,6 +4439,7 @@ dependencies = [
  "reqwest",
  "serde",
  "serde_json",
+ "tokio",
  "toml",
 ]