Add ESLint + Prettier config to frontend

[KonstantinMB]

The README's "Code conventions" section currently says "match surrounding code style" — that's fine for a solo project but breaks down with external contributors. ESLint + Prettier configs let us auto-format and auto-lint instead of arguing about style in PR reviews.

What to do

• Add frontend/eslint.config.js (flat config, ESLint 9+) with:
  • @typescript-eslint/recommended
  • eslint-plugin-react-hooks recommended
  • eslint-plugin-jsx-a11y basic accessibility rules
• Add frontend/.prettierrc — match the existing code style (look at a few .tsx files first; pick single vs double quotes, indent size, trailing commas to match what's already there)
• Add npm run lint (runs ESLint, fails on errors) and npm run format (runs Prettier --write) scripts
• Run npm run format once and commit the resulting reformat in a separate commit from the config addition, so the diff stays reviewable
• Update README "Code conventions" section to mention both commands

Acceptance criteria

• cd frontend && npm run lint passes on the current codebase
• cd frontend && npm run format reformats consistently
• No new lint warnings introduced by the configs themselves
• README mentions both commands
• Two commits: one adding configs, one applying formatting (makes the diff much easier to review)

Bonus (optional)

• Add a lint-staged + husky pre-commit hook so contributors can't easily commit unformatted code. If you do this, mention it in README.

Scope

Touches 3-4 config files + likely auto-format diffs across frontend/src/. Estimated time: 2-3 hours.

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