perf(demo-admin): memoize dashboard derivations

[Kuhai9801]

Summary

• memoize demo-admin table column configs so table sorting work is not invalidated by unrelated parent state changes
• memoize active preset, selected row, campaign assignment, and validation summary/group derivations
• reduce validation grouping from repeated filter/sort passes to one sorted pass grouped by severity

Closes #943

Performance note

The hotspot was dashboard parent/selection state recreating column arrays and validation derived data on each render, which forced AdminDataTable sorting memoization and validation grouping to rerun even when the table data or issue list was unchanged. This keeps those derived values stable while preserving the existing empty, populated, and validation-result behavior.

Validation

• git diff --check