Fix mobile lookup frame sizing

[maggo83]

Fixes two remaining narrow-screen issues in the online visual guide.

• Preserve the BIP39 word-list page aspect ratio after the Step 4 flow stacks vertically, so page frames and column highlighters end at the rendered page edge.
• Use CS for checksum marker cells on phones, where the full label cannot fit; desktop continues to show checksum.

Validation:

• offline-package/verify.py
• git diff --check
• Browser checks at 320px in English and German: page/highlighter geometry matches, checksum marker has no overflow, and the viewport stays within its client width.
• Desktop browser regression check at 1440px.