Divider mode control, segmented toggles, wall end caps, mobile trace fixes

[jaak0b]

Batch of owner-requested improvements:

• Dividers on the Manual Bin and Screw Entry tabs move out of More options into an always-visible three-state control (None, Grid, Custom). Grid mode is declarative: the two count fields apply immediately and regenerate when the bin footprint changes; switching to Custom seeds the wall editor from the grid, removing the old Apply-replaces-walls flow.
• All toggle button groups app-wide restyle as connected segmented controls (one outline, internal dividers, filled selected segment) via a single global CSS block.
• The divider wall minimum compartment gap drops from 8 mm to 2 mm.
• Free-angle divider walls get round end caps at interior endpoints, so walls meeting at any angle weld without miter notches. Boundary ends and even-grid dividers are geometrically unchanged (pinned by the existing volume assertion).
• The trace step's floating action island counter-scales against browser pinch zoom on phones so it keeps a constant on-screen size instead of growing over the photo. Owner phone check still owed for this one.

Build green, 1193 tests pass.

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