fix(ui): slim the Overview executions-by-type bars

[vybe]

What

The Agent Detail → Overview tab's Executions by type stacked bar chart rendered each day as a full-width column with only a 1px gap. On a busy agent (e.g. mostly-scheduled work) that reads as a solid wall of cyan rather than a chart.

This is a minimal, presentation-only tweak in StackedBarChart.vue:

• Cap each bar at max-w-[56px] and center it inside its column (items-center), so bars are slimmer and breathe.
• The empty-day baseline tick gets the same cap so it lines up with the bars instead of spanning full width.
• Soften the top corner (rounded-t-sm → rounded-t-md).

The column stays flex-1, so per-day spacing, the full-width hover hit-area, and the tooltip are all unchanged. Wider windows (14d/30d) already thin naturally; the cap only matters at 7d.

Scope

• 1 file, 3 class changes. No JS, no props, no OverviewPanel change, no backend.
• No behavior/data change — purely visual.

Before / after

Before: full-width columns, 1px gap → wall of color.
After: bars capped at 56px, centered, softer top.

(throwaway comparison mockup reviewed locally; 44px was too sparse, 56px chosen)

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