Issue 31-39B: Add custody analytics dashboard and printable charts

[gacurl]

Summary

Adds a lightweight offline custody analytics dashboard using the trusted 31-39A analytics layer.

Related Issue

Closes #1182

Changes

• Adds Reports → Custody Analytics
• Adds Measure, Group By, and Chart Type workflow
• Adds dependency-free bar, histogram, accountability, and line visualizations
• Adds synchronized dependent selectors
• Rejects unsupported combinations server-side
• Adds browser-print styling and operator documentation

Verification

• Dashboard route tests pass
• Custody analytics/report regression tests pass
• Selector synchronization regression tests pass
• Unsupported combinations return HTTP 400
• Docker rebuild completed
• Incognito operator workflow verified
• Browser print preview verified
• git diff --check

Notes

31-39A remains the single analytics source. No schema, dependency, event, custody, persistence, PDF embedding, external service, or internet requirement was added.