Squashed from 2 author commits:
- a1017d02 initial fix: flex:0 0 auto on all 5 chip wraps
- bf54ba50 Copilot review fix-up: consolidate into single rule
Closes#2740. CSS-only, no JS changes. Default-width layout unchanged,
only affects narrow-viewport overflow regime via composer-left's existing
overflow-x:auto.