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

mise.toml

@@ -0,0 +1,6 @@
+[tools]
+prek = "0.3.2"
+
+[settings]
+# Disable automatic trust prompts - we trust this config
+yes = true