Add ESLint and Prettier to frontend with full codebase reformat

[gauthierpiarrette]

What

Add ESLint, Prettier, and eslint-plugin-svelte to the frontend and reformat the
entire codebase to enforce consistent style. A new CI step runs npm run lint
on every push and PR so formatting and lint violations are caught automatically.

Why

The frontend had no linting or formatting tooling. Code style was inconsistent
across components (trailing commas, quote style, line length, attribute wrapping).
The Python backend already enforces style via Ruff; the frontend now has parity.

How to test

Run cd frontend && npm run lint and verify it exits cleanly with no errors.
Run cd frontend && npm test and verify all tests still pass.

Checklist

• cd frontend && npm run lint passes (ESLint + Prettier)
• cd frontend && npm test passes
• No functional changes to any component
• CI updated to run frontend lint step