Add responsive multi-mode quiz practice interface

[apple246680]

Summary

• create a responsive single-page quiz interface with light and dark themes
• implement free practice, custom range, and mock exam flows with bookmarks, progress, and scoring
• add progress tracking, jump navigation, and range selection support for the question bank

Testing

• Not run (not applicable)

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https://chatgpt.com/codex/tasks/task_e_6904a708f2788320922e0e5eecd66927

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Prevent starting quiz before question data loads

The mode handlers assume the question bank is already populated when the buttons are clicked. Because the buttons are wired up before loadQuestions() resolves, a user on a slow or failed fetch can start a quiz while state.questions is still empty, which renders a blank quiz and yields Infinity when finishExam divides by total (0 questions). Consider disabling the mode buttons until the fetch completes or guard these functions so they wait for the questions to load before copying them into state.currentQuestions.

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