ui: collapse .config-grid to single column on mobile
Smoke test of the prior overflow hotfix narrowed the residual
overshoot on /config from +60 -> +27 px, but a fixed
grid-template-columns: 150px 1fr with auto-min-width on the 1fr
track was still forcing two ~207 px tracks at 375 viewport.
Mobile media block now sets:
- grid-template-columns: 1fr (single column, label above input)
- min-width: 0 on grid children so long content cannot push tracks
- gap: 4px 12px (vertical 4 px between label and input pair)
After: /config measures 0 px overflow at 375x812.
Refs #19