a11y(settings): announce theme updates politely

[emirykl]

Summary

• add a visually hidden polite, atomic live region for theme-setting updates
• debounce announcements by 300 ms so rapid changes announce only the latest selection
• keep the live region empty on mount
• preserve existing theme persistence and application behavior

Test evidence

• npm test -- --no-watchman --runInBand src/components/__tests__/ThemeToggle.test.tsx src/app/settings/page.test.tsx — passed, 10/10 tests
• focused coverage — passed with 100% statements, branches, functions, and lines for ThemeToggle.tsx
• focused ESLint — passed
• npm run build — passed
• git diff --check — passed
• full Jest baseline: 75 suites passed and 2 unrelated suites failed in existing API-key and theme SSR tests
• repository lint/typecheck retain unrelated pre-existing errors in service edit, useApi, apiClient tests, and validateNumber tests

Fixes #411

Verification

• PASS: focused settings tests — 10/10
• PASS: focused ESLint
• PASS: ThemeToggle coverage — 100% statements, branches, functions, and lines
• PASS: npm run build
• PASS: git diff --check
• BASELINE FAILURE: full Jest — 75 suites passed, 2 unrelated suites failed (API keys and theme SSR tests)
• BASELINE FAILURE: repository lint — 3 existing errors in service edit/useApi files
• BASELINE FAILURE: typecheck — 5 existing errors in apiClient/validateNumber tests

Diagnostics

Recovered command failures: /bin/zsh -lc 'npm ci --ignore-scripts' (255), /bin/zsh -lc 'npm test -- --no-watchman --runInBand src/components/__tests__/ThemeToggle.test.tsx --coverage --collectCoverageFrom=src/components/ThemeToggle.tsx' (1), /bin/zsh -lc 'npm run typecheck' (2), /bin/zsh -lc 'npm run lint' (1), /bin/zsh -lc 'npm test -- --no-watchman --runInBand' (1)