Refine Quizify UI and strengthen E2E coverage

[not-wowmagic]

Summary

• remove the unused dark theme controls and emoji-driven presentation
• refine mobile runner actions, sizing, persistence, generation cancellation, retry, and input feedback
• preserve the active quiz during regeneration and add safer history deletion
• replace eligible client navigation with real links
• add mobile, loading/error, regeneration, and character counter E2E coverage
• fix the React hooks lint error in the completion effect

Validation

• npm run lint
• npm run typecheck
• npm test (116 tests)
• npm run test:e2e (77 tests across desktop and mobile Chromium)

[netlify]

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