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/message/server/message_user.rs

@@ -20,12 +20,12 @@ impl MessageHandler<ServerMessage> for MessageUser {
         let user_message = UserMessage::new(
             id,
             timestamp,
-            username.clone(),
+            username,
             message_content,
             new_message,
         );
 
         info!("[MessageUser] User message received:{:?}", user_message);
-        user_message.print()
+        user_message.print();
     }
 }