docs: document CSP-safe styling convention for frontend

[brite-side0]

Summary

docs: document CSP-safe styling convention for frontend

What was solved

#1670 — docs(frontend): document CSP-safe styling conventions (no inline styles)

Document the CSP-safe styling convention for the frontend: explain that inline style props are stripped by the Content-Security-Policy (per commits 5bd5e51 and e80a15b), mandate CSS classes/CSS modules instead, note the current lack of automated enforcement and whether an ESLint rule (e.g. react/forbid-dom-props for style) should back it up, and add a short pointer from CONTRIBUTING.md to the rule.

Addressed:

• Changed: frontend/README.md, frontend/docs/styling.md, CONTRIBUTING.md
• Document the convention in frontend/README.md (per docs(frontend): add frontend/README.md for local dev onboarding #1668) or a dedicated frontend/docs/styling.md
• Explain WHY (CSP strips/blocks inline styles) so the rule isn't silently reverted
• State the required pattern: use CSS classes / CSS modules instead of style={{...}} props for anything affecting rendering

Changes

• CONTRIBUTING.md (modify)
• frontend/README.md (create)
• frontend/docs/styling.md (create)

Approach

1. Read frontend/README.md (or create frontend/docs/styling.md if README is absent) and CONTRIBUTING.md to match existing heading/link conventions.
2. Add a 'Styling' section documenting the CSP-safe convention: explain that inline style props are stripped by the Content-Security-Policy (refs 5bd5e51, e80a15b), so rendering-affecting styles must use CSS classes / CSS modules instead of style={{...}}.
3. Note the current lack of automated enforcement and recommend an ESLint rule (e.g. react/forbid-dom-props for style) as a future option, without adding tooling or dependencies.
4. Add a short pointer from CONTRIBUTING.md to the styling rule.
5. Keep changes documentation-only; do not refactor components or migrate existing inline styles.

Issues

Closes #1670