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-utils/src/singleton.rs

@@ -80,6 +80,9 @@ pub fn try_acquire(app: &str) -> std::io::Result<Acquire> {
         .read(true)
         .write(true)
         .create(true)
+        // Never truncate on open: an existing lock file may be held by a live
+        // instance. We only clear it (`set_len(0)`) *after* winning the lock.
+        .truncate(false)
         .open(&path)?;
 
     match file.try_lock() {