Add prek pre-commit hooks via mise, fix formatting and clippy warnings

- Add mise.toml (prek 0.3.2) and .pre-commit-config.yaml with hooks for
  trailing whitespace, end-of-file, YAML check, merge conflicts, large files,
  cargo fmt, cargo clippy (-D warnings), and unit tests
- Add prek/mise recipes to justfile
- Run cargo fmt to fix formatting issues
- Fix all clippy warnings: collapse nested if statements, derive Default for Avatar

src/bot/implementation.rs

@@ -4,10 +4,10 @@ use std::{future::Future, pin::Pin};
 
 use mxlink::matrix_sdk::Room;
 use mxlink::matrix_sdk::media::{MediaFormat, MediaRequestParameters};
+use mxlink::matrix_sdk::ruma::api::client::profile::{AvatarUrl, DisplayName};
 use mxlink::matrix_sdk::ruma::{
     MilliSecondsSinceUnixEpoch, OwnedUserId, events::room::MediaSource,
 };
-use mxlink::matrix_sdk::ruma::api::client::profile::{AvatarUrl, DisplayName};
 
 use mxlink::{
     InitConfig, LoginConfig, LoginCredentials, LoginEncryption, MatrixLink, PersistenceConfig,