clippy: clean up all warnings

Mostly `cargo clippy --fix`: needless borrows, redundant field names,
match -> if let, io::Error::other, Option::filter/map, let chains, c""
literals. By hand: drop unused impl lifetimes, a type alias for the
font preload map, Iterator::find in next_buffer, contains_key, and
allow new_ret_no_self where constructors intentionally return the
boxed/generic wrapper type.

No functional change; tests and release build unchanged.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/utils/desktop.rs

@@ -45,10 +45,10 @@ impl Desktop {
                     .map(|x| {
                         x.split(';')
                             .map(|y| y.trim().to_string())
-                            .filter(|z| z != "")
+                            .filter(|z| !z.is_empty())
                             .collect()
                     })
-                    .unwrap_or_else(|| vec![]),
+                    .unwrap_or_else(std::vec::Vec::new),
             }),
             None => Err(Box::new(io_error::new(
                 ErrorKind::NotFound,