frontend: real-time chart component improvements (incremental updates & decimation)

[ayushkrtiwari]

Goal

Make cauweb charts handle frequent updates without React re-render storms.

Tasks

• Implement append-only drawing for live series (OffscreenCanvas / canvas layer) in LiveChart component.
• Add client-side decimation strategy (window aggregation / last/avg/max).
• Expose chart-level "quality" (high/med/low) toggle in UI.
• Ensure charts accept replay data for manual review and exported snapshots.

Acceptance criteria

• Live charts remain responsive at target tick rates (verified with synthetic feed).