New Params panel (Log viewer / Params / Metrics / Dashboards) declares a
bare param via POST /api/params, mirroring Metrics' declare-then-list
pattern including inline 400 error surfacing. Lifted the params list from
LogPreview's own effect into App.tsx so declaring a param there updates
the Columns picker immediately, without a page reload.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o
