fix: make navbar scroll progress reach page end

[saurabhhhcodes]

Summary

• mount the existing scroll progress bar in the application shell
• calculate progress from the active scrolling element
• clamp the value so the bar reaches exactly 100% at the bottom

Validation

• git diff --check
• npm run build (passes with pre-existing lint warnings)
• CI=true npm test -- --watchAll=false --runInBand (3 tests passed)

Closes #480

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