v0.3.1

Patch release.

Tooling (permanent fix for the recurring fmt drift):
- rustfmt.toml pins edition / reorder_imports / max_width explicitly.
- .githooks/pre-commit runs cargo fmt --check on staged Rust and
  aborts on failure, so editor/agent on-save formatting mismatches
  can no longer reach CI.

Web fix:
- Hide the install-dialog copy button on insecure origins.

No functional changes to the search engine.