style: crank clippy to pedantic+nursery and fix all lints

Enable the full clippy all + pedantic + nursery groups as workspace warnings,
with a small documented allow-list for the pedantic/nursery lints this project
intentionally does not follow (rustdoc-style requirements, deliberate numeric
casts in the protocol/UI layers, and a few noisy nursery lints). Also enable
the rust unused_lifetimes and unsafe_op_in_unsafe_fn lints.

Fixed every resulting warning (527 -> 0) mechanically per clippy's suggestions:
inline format args, const fns, item ordering, let-else, redundant clones,
map_or, cast_lossless, Eq derives, associated-fn conversions for methods that
never used self, and removing unnecessary Result wrappers whose call sites could
never propagate an error. No runtime behavior changed; cargo fmt applied.

cargo clippy --workspace --all-targets: 0 warnings. All tests pass.

soulseek-rs-lib/src/dispatcher.rs

@@ -10,12 +10,13 @@ pub struct MessageDispatcher<Op> {
 }
 
 impl<Op> MessageDispatcher<Op> {
-    pub fn new(
+    #[must_use]
+    pub const fn new(
         owner_name: String,
         sender: Sender<Op>,
         handlers: Handlers<Op>,
     ) -> Self {
-        MessageDispatcher {
+        Self {
             owner_name,
             sender,
             handlers,