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/components/LogPreview.tsx

@@ -1,9 +1,5 @@
 import { useEffect, useRef, useState } from 'react'
-import {
-  fetchParamDeclarations,
-  metricPrimitiveToDisplayString,
-  type LogEntry,
-} from '../lib/api'
+import { metricPrimitiveToDisplayString, type LogEntry, type ParamDeclaration } from '../lib/api'
 import { HelpIcon } from './HelpIcon'
 
 /// localStorage key for LogPreview's selected param columns. Namespaced
@@ -89,25 +85,10 @@ function ColumnPicker({
   )
 }
 
-export function LogPreview({ logs }: { logs: LogEntry[] }) {
-  const [paramNames, setParamNames] = useState<string[]>([])
+export function LogPreview({ logs, params }: { logs: LogEntry[]; params: ParamDeclaration[] }) {
+  const paramNames = params.map((d) => d.name)
   const [visibleParamColumns, setVisibleParamColumns] = useState<string[]>(loadVisibleParamColumns)
 
-  useEffect(() => {
-    let cancelled = false
-    fetchParamDeclarations()
-      .then((declarations) => {
-        if (!cancelled) setParamNames(declarations.map((d) => d.name))
-      })
-      .catch(() => {
-        // params API not reachable yet; column picker just stays empty until
-        // the next successful fetch.
-      })
-    return () => {
-      cancelled = true
-    }
-  }, [])
-
   const visible = new Set(visibleParamColumns)
 
   function toggleColumn(name: string) {