fix: responsive design overhaul with 5 progressive breakpoints (#108)

- Remove old conflicting breakpoints (820px, 540px, 680px)
- Add L (≤1199px), M (≤899px), S (≤679px), XS (≤479px), XXS (≤374px)
- Platform strip: desktop 2-col → tablet 3-col → mobile 2-col (no more 5-col scroll)
- Topbar degrades progressively: shrink → stack → hide subtitle
- Form sections collapse at 899px instead of 680px
- All touch targets ≥42px on mobile
- Pure CSS changes, no HTML/JS modifications

Co-authored-by: Claude <noreply@anthropic.com>

Author: Pigbibi