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/widgets/date.rs

@@ -29,7 +29,7 @@ impl Date {
     }
 }
 
-impl<'a> Widget for Date {
+impl Widget for Date {
     fn get_dirty(&self) -> bool {
         self.dirty
     }
@@ -39,11 +39,8 @@ impl<'a> Widget for Date {
     }
 
     fn event(&mut self, event: &Event) {
-        match event {
-            Event::NewMinute => {
-                self.dirty = true;
-            }
-            _ => (),
+        if let Event::NewMinute = event {
+            self.dirty = true;
         }
     }