test/build: add eslint-plugin-react-hooks exhaustive-deps rule

[jamesa94]

Background

The ESLint configuration in frontend/.eslintrc.cjs includes plugin:react-hooks/recommended, which enables the react-hooks/exhaustive-deps and react-hooks/rules-of-hooks rules. However, these rules are set to warn by default in the recommended preset, meaning missing dependencies in useEffect (like the bug in PR-F03 where CodeDashboard has a stale closure) are only warnings, not build-blocking errors.

This allows subtle bugs to slip through CI and reach production.

Changes

Added explicit rule configuration to elevate both React Hooks rules from warn to error:

• react-hooks/exhaustive-deps: error (catches missing dependencies in useEffect/useCallback/useMemo)
• react-hooks/rules-of-hooks: error (catches hooks called conditionally or outside components)

Before

rules: {
  react-refresh/only-export-components: off,
  @typescript-eslint/no-unused-vars: [error, { argsIgnorePattern: ^_ }],
},

After

rules: {
  react-refresh/only-export-components: off,
  @typescript-eslint/no-unused-vars: [error, { argsIgnorePattern: ^_ }],
  react-hooks/exhaustive-deps: error,
  react-hooks/rules-of-hooks: error,
},

Notes

After enabling this rule, running npm run lint will likely surface existing violations (e.g., the CodeDashboard.tsx useEffect with a stale loadSessions closure). Those should be fixed in follow-up PRs using useCallback wrappers or proper dependency arrays. This PR is intentionally minimal: it only changes the ESLint config, not the code that violates it.