fix(ci): unblock frontend matrix after dependabot major bumps

[rxf-sys]

The dependabot frontend-dependencies group bump (#17) jumped four eslint plugins, vite, vitest, typescript and react in one shot. Several of those hops broke the CI matrix in ways the bot couldn't see:

• eslint 8 → 10: eslint-plugin-react@7.37.5 still peers on eslint ^9.7, so npm ci failed with ERESOLVE on every run. Pin eslint to ^9.39 — the newest line all four plugins accept.

• @testing-library/react 16 promoted @testing-library/dom from a bundled dep to a peer. It was missing from devDependencies, so tsc failed with TS2305 on every screen / fireEvent / waitFor import. Add @testing-library/dom.

• typescript 6 stopped silently allowing CSS side-effect imports, so main.tsx failed with TS2882 on the three styles/*.css imports. Add the standard src/vite-env.d.ts shim (/// <reference types="vite/client" />).

• eslint 9 dropped .eslintrc.* — migrate to the flat eslint.config.js. no-undef is turned off because @typescript-eslint already covers it, and the old config implicitly relied on that via the recommended preset.

• eslint-plugin-react-hooks 4 → 7 added experimental rules (set-state-in-effect, refs) that flag standard ref/effect patterns as errors. Pin back to ^5.2 — the last line whose recommended matches the rule set the project was written against. v6/v7 can be revisited deliberately, not via the bot's grouped bump.

Also drops two now-redundant // eslint-disable-next-line no-console comments in Drawer.tsx that became "Unused eslint-disable" warnings.

Verified locally: backend ruff + 54 pytest, frontend tsc -b + eslint

• 31 vitest + vite build all green. npm ci reproduces cleanly from the new lockfile.