fix: resolve cargo fmt and clippy errors breaking CI

[careck]

Summary

• All CI runs were failing at the cargo fmt --check step — 3 formatting diffs in export_tests.rs and notes.rs (commands)
• Behind that gate, cargo clippy -- -D warnings had 35 errors across all 3 workspace crates (krillnotes-core, krillnotes-rbac, krillnotes-desktop), accumulated as newer Rust stable versions added lints
• Fixed all issues: mechanical cleanups (unnecessary casts, redundant closures, needless borrows, extend→append, format args, map_or→is_none_or, manual clamp, derivable impl, const thread_local, match→if let), doc formatting, and #[allow(...)] for architectural choices (type_complexity, too_many_arguments)

Test plan

• cargo fmt --check passes
• cargo clippy --workspace -- -D warnings passes (0 errors)
• cargo test -p krillnotes-core — all tests pass