Add Params declaration UI (ticket 034)

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

webapp/src/lib/api.ts

@@ -273,6 +273,10 @@ export function fetchParamDeclarations(): Promise<ParamDeclaration[]> {
   return getJson('/api/params')
 }
 
+export function declareParam(request: ParamDeclaration): Promise<ParamDeclaration> {
+  return postJson('/api/params', request)
+}
+
 export function fetchMetricDeclarations(): Promise<MetricDeclaration[]> {
   return getJson('/api/metrics')
 }