fix: clippy single_match in heph-tui + add clippy pre-commit hook

CI on main failed on a clippy::single_match lint in the heph-tui sidebar
key handler. Rewrite as `if let`. Also add a `cargo clippy -D warnings`
prek hook mirroring cargo-fmt, so lints are caught locally before CI
(prek previously only ran cargo fmt).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/changelog.d/+clippy-prek-hook.infra.md

@@ -0,0 +1 @@
+Add a `cargo clippy -D warnings` pre-commit hook so lints are caught locally before CI.

docs/changelog.d/+fix-clippy-single-match.bugfix.md

@@ -0,0 +1 @@
+Fix `clippy::single_match` lint in heph-tui sidebar key handling (CI failure on main).