Ark0N's PR #453 review: nothing gated this feature to desktop even
though the design called for it (two 240px min-width panes plus the
divider need ~486px, and the divider has no touch handlers), and
showSplitButton was a SYNCED setting, so turning it on at a desk also
put the button in the phone header.
- Hard-hide .btn-split on phones in mobile.css regardless of the
setting, matching the other desktop-oriented header buttons in the
same @media (max-width: 599px) block.
- Move showSplitButton into settings-ui.js's per-device displayKeys
set and drop it from SettingsUpdateSchema entirely, matching the
showFileViewerButton/skin precedent (CLAUDE.md's "per-device keys
... must NOT be added to SettingsUpdateSchema" rule) — a desktop
opt-in must never sync onto a phone that never asked for it. Removes
the now-invalid server-round-trip test for the setting.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
