[Design P2] Improve mobile touch targets and responsiveness

[AnilChinchawaleXDC]

Source: DESIGN-AUDIT.md

Problem: Bottom nav touch targets are small (icon + 9px label). No swipe gestures. Stat values drop to 20px on mobile.

Actions:

1. Increase bottom nav touch targets to 44px minimum
2. Increase mobile stat values for readability
3. Consider adding swipe gestures
4. Test on actual mobile devices

Risk: Poor mobile UX, accessibility issues.

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Mirrored from Paperclip OPEA-373 (https://agi.openscan.ai/OPEA/issues/OPEA-373)