Fix clippy warnings and a stale test assertion surfaced by check.yml

check.yml runs clippy -D warnings and cargo test for the first time in
this repo's history, surfacing pre-existing lint debt across all four
workspace crates plus one stale test assertion (breadpad-shared's
Palette default-background test still expected bread-theme's old
#1e1e2e; the upstream design system moved to #0c0c0c a while back and
nothing caught the drift since tests never ran in CI).

All fixes are mechanical: needless returns/closures/borrows, manual
Default impls replaceable by #[derive], map_or -> is_some_and, manual
range checks -> RangeInclusive::contains, a non-idiomatic sort_by ->
sort_by_key, an overly complex inline type given an alias, and moving
a function that ended up defined after its own test module. The one
exception is NoteType::from_str, which clippy flags for shadowing
std::str::FromStr's name — left as `#[allow(...)]` with a comment
rather than renamed, since it has 30+ call sites across every crate
in the workspace and returns Self directly rather than Result.

breadpad-shared/src/scheduler.rs

@@ -176,7 +176,7 @@ pub(crate) fn parse_next_from_rrule(rrule_str: &str, default_morning: &str) -> O
             } else {
                 (now.date_naive() + chrono::Duration::days(1)).and_time(fire_time)
             };
-            return Some(local_naive_to_utc(naive));
+            Some(local_naive_to_utc(naive))
         }
         "WEEKLY" => {
             use chrono::Datelike;
@@ -204,7 +204,7 @@ pub(crate) fn parse_next_from_rrule(rrule_str: &str, default_morning: &str) -> O
             };
             let target_date =
                 (now.date_naive() + chrono::Duration::days(days_ahead)).and_time(fire_time);
-            return Some(local_naive_to_utc(target_date));
+            Some(local_naive_to_utc(target_date))
         }
         _ => None,
     }