feat: upgrade to React 19, Vite 8, ESLint 10, TypeScript 6, React Router v7

[willwillis]

Summary

Major frontend dependency upgrades — all breaking version bumps deferred from the v1.5.0 safe-update pass.

• React 18 → 19, react-dom 18 → 19
• react-router-dom 6 → 7
• Vite 7 → 8, @vitejs/plugin-react 4 → 6
• TypeScript 5 → 6
• ESLint 8 → 10, @eslint/js 9 → 10
• @typescript-eslint/eslint-plugin + parser 6 → 8 (fixes 6 high CVEs from minimatch ReDoS)
• eslint-plugin-react-hooks 4 → 7
• @types/react + react-dom 18 → 19
• @testing-library/react 14 → 16
• jsdom 23 → 29
• Bump version to 1.6.0

Fixes

• Added src/vite-env.d.ts with /// <reference types="vite/client" /> — required by TypeScript 6 for CSS side-effect imports (import './index.css')

Test plan

• npm run build — passes with Vite 8
• npm run lint — passes with ESLint 10
• npm run test -- --run — 3/3 pass
• npm run test:coverage -- --run — coverage runs clean
• CI pipeline passes on all Python matrix versions

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.52%. Comparing base (c1ca5b3) to head (6b75aac).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #10   +/-   ##
=======================================
  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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