#92 mobile: PR head→base breaks inside branch names

What

Mobile PR cards allow anywhere-wrapping in the head→base cell, so a branch pair breaks inside a word:

m6/submit-buttons→mast
er

Reproduced

/pulls/index.html at 390 px.

Impact

Looks broken and can be misread; branch names are tokens.

Fix

Stack head/base on narrow screens (two lines with the arrow), or keep one line with white-space:nowrap inside a scrollable cell. Never break inside a branch name. Add a responsive assertion/screenshot check.