Clear clippy 1.97 lints across bread-theme, bread-utils, bread-polkit

Toolchain drift (clippy 0.1.97): pre-existing on main, all mechanical
and behaviour-preserving.

- bread-theme/gtk.rs: `type AppCssBuilder` alias for the repeated
  `Rc<dyn Fn(&Palette) -> String>` (type_complexity ×4).
- bread-theme/lib.rs: `sort_by_key(|..| Reverse(len))` (unnecessary_sort_by).
- bread-theme/output.rs: `io::Error::other`; struct-init in a test.
- bread-utils/singleton.rs: explicit `.truncate(false)` on the lock file
  open — we only clear it after winning the lock (suspicious_open_options).
- bread-polkit/identity.rs: elide `pick_user` lifetimes.

bread-polkit/src/identity.rs

@@ -37,7 +37,7 @@ pub fn users_from_uids(uids: &[u32], passwd: &str) -> Vec<UnixUser> {
 }
 
 /// Prefer the process's own uid when it is in `users`, otherwise the first.
-pub fn pick_user<'a>(users: &'a [UnixUser], current_uid: Option<u32>) -> Option<&'a UnixUser> {
+pub fn pick_user(users: &[UnixUser], current_uid: Option<u32>) -> Option<&UnixUser> {
     if let Some(uid) = current_uid {
         if let Some(user) = users.iter().find(|u| u.uid == uid) {
             return Some(user);