fix: add dashboard line chart labels and hover tooltips

[newbish]

Summary

• add a small set of x-axis labels to the dashboard line graphs so time context is visible at a glance
• add point markers and hover targets so each bucket shows its timestamp and value on mouseover
• keep the existing chart layout, while reusing shared line-chart geometry helpers for ticks, points, and tooltip regions

Test Plan

• cd apps/dashboard && npm run typecheck
• cd apps/dashboard && npm test
• cd apps/dashboard && npm run build

[github-actions]

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