Add opt-in pre-commit hooks

[Sikkra]

Summary

• Adds an opt-in Lefthook pre-commit config with staged-file scoped frontend and Rust checks.
• Adds a root README section documenting install, what runs, and emergency bypass flags.
• Adds a frontend typecheck script and small compatibility fixes so the frontend hook passes.

Verification

• npm run build
• npm run typecheck
• npx --yes lefthook validate
• npx --yes lefthook run pre-commit
• git diff --check

Closes #77