Set up code formatting and linting

### Summary
Add automated formatting tooling for the codebase: Prettier for JS/CSS and a formatter for Django HTML templates. Includes initial reformatting pass, CI integration, and pre-commit hooks.

### Motivation
The codebase has inconsistent formatting across CSS, JS, and HTML files, making diffs noisy and code review harder. Automated formatters eliminate style debates and keep contributions consistent.

### Scope
- **Prettier**: Configure for JS and CSS files (`.prettierrc`, `.prettierignore`)
- **Django template formatter**: Find and configure a tool for HTML template formatting
- **Pre-commit hooks**: Add formatters to `.pre-commit-config.yaml`
- **CI**: Add formatting check step to GitHub Actions
- **Just commands**: Add `just format` / `just lint` shortcuts
- **Initial reformat**: Run formatters across all existing files
- **Docs**: Update README with vendor JS policy and any setup required for both existing and future devs.