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.toml

@@ -45,3 +45,4 @@ reqwest = { version = "0.12", features = ["blocking", "json"] }
 # GUI for the setup wizard
 iced = { version = "0.13", features = ["tokio"] }
 iced_fonts = { version = "0.1", features = ["bootstrap"] }
+tokio = { version = "1.52.1", features = ["rt"] }