Split timeline width across lanes instead of stacking overlapping events

The CSS gives every .calendar-event `left: 8px; right: 8px`, and the
positioning code only ever overrode `left` (by 16px per column). So two
events occupying the same slot both stayed full-width to the right edge
and the later one simply covered the earlier one, leaving a 16px sliver.
The column assignment was computing a correct answer that the layout then
threw away.

Reported as two yard-work chores rendering on top of each other. They sit
at byte-identical timestamps because a bucket cycle stamps every item it
activates with the same due_date, which makes this the maximally-bad case
(identical top and height, so the overlap is total).

Events now split the available width across the number of lanes in their
overlap cluster, and a lone event keeps the full-width behaviour it had.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018AwnALZL6bD4VBtATDVv3B

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