fix: Fix quiz locale matching for zh-CN

[LHN012]

Description

This fixes Simplified Chinese quiz loading in the quiz app.

The app has a Simplified Chinese translation key named zh_cn, but browsers usually report the locale as zh-CN. Because the app compared the raw locale with locale.startsWith(key), zh-CN did not match zh_cn.

This change normalizes locale values by lowercasing and replacing - with _. It also lets the loc query parameter choose the quiz translation before the quiz file is selected.

Fixes no linked issue.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Tested

• npm run build
• http://localhost:5173/?loc=zh_cn shows the Chinese quiz list
• http://localhost:5173/quiz/1?loc=zh_cn shows Lesson 1 pre-lecture quiz in Chinese
• Clicking the correct first answer advances to the second question

[LHN012]

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