Add pre-commit hooks for linting and formatting

Set up pre-commit framework with ruff (backend) and ESLint/Prettier/tsc
(frontend) hooks to catch issues locally before CI. Auto-format all
frontend files with Prettier to comply with the new check.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -8,6 +8,18 @@
 - **Merge commit** `develop` into `main` (marks deploy points).
 - Always `git pull` the target branch before merging into it.
 
+# Pre-commit Hooks
+
+This project uses [pre-commit](https://pre-commit.com/) to run linting and formatting checks before each commit.
+
+**Setup:** `pip install pre-commit && pre-commit install`
+
+**Hooks configured:**
+- **Backend:** `ruff check --fix` and `ruff format` on Python files under `backend/`
+- **Frontend:** `eslint`, `prettier --check`, and `tsc -b` on files under `frontend/`
+
+Frontend hooks require `npm ci` in `frontend/` first (they use `npx` to run from local `node_modules`).
+
 # Instructions
 
 - After completing a task, always ask the user if they'd like to commit the changes.