- Add `is_mobile` to SharedPaneState (set in build_frame). On mobile the timeline
layer-header column collapses to a minimal type-colored swatch (1:2 w:h, ~30px),
brighter/outlined when active/selected; tapping it selects the layer (handled by
the existing header-column input path, which now uses the narrower width).
Desktop headers unchanged.
- Inspector reflow: the sheet overlaps the stack by default, but when the selected
pane would be covered by the sheet, the stack reflows (renders above the sheet)
so it stays visible. Restore on dismiss is automatic since reflow only changes
the render rect; manual divider moves while the sheet is up persist. Adds
stack::pane_bottom_in and inspector::target_slot.
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