Introduce vertical cost detail list

[iam-brain]

Summary

This PR now focuses only on the cost chart hover UI change: it replaces the single secondary detail line with a vertical per-model list in the chart detail area.

The branch intentionally keeps main's current data behavior. There are no scanner, decoding, CLI payload, or model-shape changes in this version.

What changed

• CostHistoryChartMenuView now shows a stacked detail block under the chart.
• The selected day still renders the existing primary summary line for date, cost, and tokens when available.
• Model breakdowns render as a vertical list with accent bars and per-row cost detail.
• The list is capped at four visible rows.
• The menu reserves detail height based on the busiest loaded day so hover movement does not resize the menu.

What did not change

• No changes to CostUsageModels.
• No changes to the cost scanners.
• No changes to decoding or CLI payloads.
• No pricing or total-cost behavior changes.

Verification

• pnpm check
• swift test
• ./Scripts/compile_and_run.sh

[iam-brain]

Rebased against main and pruned to reduce scope to a purely UX/UI improvement.