fix: post-React 19 cleanup — remove dead deps, wire up ESLint plugins, modernize entry

[willwillis]

Summary

Follow-up cleanup from the React 19 upgrade (PR #10). Addresses overlooked items identified during post-merge review.

Changes

🔴 Fixes

• Remove unused react-router-dom — upgraded to v7 in feat: upgrade to React 19, Vite 8, ESLint 10, TypeScript 6, React Router v7 #10 but never imported anywhere in source code. Eliminates ~7 transitive packages from the dependency tree.
• Wire up ESLint plugins — eslint-plugin-react-hooks (v7) and eslint-plugin-react-refresh (v0.5) were installed but never configured in eslint.config.js. Now enforces rules-of-hooks (error) and exhaustive-deps (warn).

🟡 Cleanup

• Idiomatic React 19 entry point — main.tsx now uses named imports (createRoot, StrictMode) instead of namespace import (ReactDOM.createRoot, React.StrictMode).
• Drop as any cast on import.meta.env — the vite-env.d.ts added in feat: upgrade to React 19, Vite 8, ESLint 10, TypeScript 6, React Router v7 #10 provides proper types; the (import.meta as any) workaround in api.ts is no longer needed.
• Consolidate duplicate vite-env.d.ts — merged root-level frontend/vite-env.d.ts (with ImportMetaEnv interface) into frontend/src/vite-env.d.ts (the one actually in tsconfig include path). Deleted the dead root-level copy.
• Remove redundant autoprefixer — Tailwind CSS v4 includes autoprefixing built-in. Removed from both package.json and postcss.config.js.
• Re-enable CI checks — tsc --noEmit and npm audit --audit-level=moderate were skipped with TODO comments; both now pass clean.

Verification

• npm run lint — passes (now with react-hooks and react-refresh rules active)
• npx tsc --noEmit — passes
• npm run test -- --run — 3/3 pass
• npm run build — passes
• npm audit — 0 vulnerabilities
• npm outdated — nothing outdated

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.52%. Comparing base (1c1d576) to head (1c08722).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #11   +/-   ##
=======================================
  Coverage   71.52%   71.52%           
=======================================
  Files           6        6           
  Lines         288      288           
=======================================
  Hits          206      206           
  Misses         82       82           

Flag	Coverage Δ
backend	71.52% <ø> (ø)

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