feat: show scroll progress on quiz pages

[saurabhhhcodes]

Summary

• show the existing scroll progress bar on /quiz/[slug] pages
• keep the current semester-page behavior unchanged
• leave the back-to-top button behavior unchanged across routes

Fixes #186

Root Cause

ProgressBar already calculates scroll progress globally, but the top progress indicator only rendered for routes beginning with /sem.

Validation

• npm run lint passes with existing warnings only
• npm run build passes
• git diff --check passes

Impact

Long quiz pages now get the same reading-progress affordance as semester content pages.

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