Stop iOS double-tap zoom on keyboard and buttons
Fixes user feedback: on iOS, pressing backspace more than once
triggered Safari's double-tap-zoom, because rapid taps on the same
element are read as a zoom gesture.
The board already set touch-action: manipulation, but the on-screen
keyboard keys and general buttons did not, leaving them exposed.
Add touch-action: manipulation to .key and .btn. This disables only
the double-tap-zoom delay on those controls; pinch-to-zoom still
works, so no accessibility loss. A viewport-meta fix
(maximum-scale/user-scalable) was rejected — iOS ignores it since
iOS 10 and it would disable pinch-zoom entirely.