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

src/util.rs

@@ -1,6 +1,4 @@
-use iced::futures::channel::oneshot;
-
-/// Run a blocking closure on a thread pool thread and await its result.
+/// Run a blocking closure on the tokio blocking thread pool and await its result.
 /// Used to offload blocking work (HTTP, disk, process spawning) without
 /// stalling the iced event loop.
 pub async fn async_blocking<T, F>(f: F) -> T
@@ -8,11 +6,9 @@ where
     T: Send + 'static,
     F: FnOnce() -> T + Send + 'static,
 {
-    let (tx, rx) = oneshot::channel();
-    std::thread::spawn(move || {
-        let _ = tx.send(f());
-    });
-    rx.await.expect("blocking task panicked")
+    tokio::task::spawn_blocking(f)
+        .await
+        .expect("blocking task panicked")
 }
 
 /// Open a native folder picker dialog and return the chosen path, or None if