fix(web): show sidebar counts in the caret slot, caret on hover - #61

The sidebar spent a whole column on the count of hidden work and process-group members, next to an otherwise empty caret slot.

• Column removed: the count column is gone, and its width goes back to the other columns. Wide titles now span model / issue.
• Count in the caret slot: collapsed threads and process groups show their count there, in 10px tabular digits, capped at 99+ with the full count in the tooltip and the toggle's label.
• Caret on hover: row hover or toggle focus shows the caret instead, using CSS only. Open rows show the downward caret, and toggles without a count keep the caret.
• Wider slot: the caret slot is 1.5rem wide so two digits fit. Carets and counts align at every depth.

Closes #58

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