Fix oversized mobile landing screenshots

[sarthakagrawal927]

Summary

• preserve screenshot aspect ratios explicitly instead of letting declared image height dominate mobile layout
• compact the hero device and four product chapters into a readable alternating mobile rhythm
• add 390, 768, and 1440 browser evidence plus a passing Fleet design-review receipt

Verification

• pnpm check
• zero horizontal overflow at 390, 768, and 1440 pixels
• Impeccable critique: 35/40 normalized (28/32 applicable)
• technical audit: 18/20
• zero unresolved P0/P1 findings

No deployment is included.

Closes #14