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/App.tsx

@@ -3,10 +3,20 @@ import { NavMenu } from './components/NavMenu'
 import { FilterBar } from './components/FilterBar'
 import { LogTimeline, computePresetRange, type TimeRange, type TimeRangePresetId } from './components/LogTimeline'
 import { LogPreview } from './components/LogPreview'
+import { Params } from './components/Params'
 import { Metrics } from './components/Metrics'
 import { Dashboards } from './components/Dashboards'
 import { Alerts } from './components/Alerts'
-import { fetchLogs, fetchStats, fetchTimeline, type LogEntry, type Stats, type TimelineBucket } from './lib/api'
+import {
+  fetchLogs,
+  fetchParamDeclarations,
+  fetchStats,
+  fetchTimeline,
+  type LogEntry,
+  type ParamDeclaration,
+  type Stats,
+  type TimelineBucket,
+} from './lib/api'
 import { DocsNavProvider } from './lib/docsNav'
 
 const POLL_MS = 2000
@@ -14,6 +24,7 @@ const POLL_MS = 2000
 export default function App() {
   const [stats, setStats] = useState<Stats | null>(null)
   const [logs, setLogs] = useState<LogEntry[]>([])
+  const [params, setParams] = useState<ParamDeclaration[]>([])
   const [buckets, setBuckets] = useState<TimelineBucket[]>([])
   const [range, setRange] = useState<TimeRange | null>(null)
   const [activePreset, setActivePreset] = useState<TimeRangePresetId | null>('all')
@@ -30,6 +41,18 @@ export default function App() {
     rangeRef.current = range
   }, [range])
 
+  async function loadParams() {
+    try {
+      setParams(await fetchParamDeclarations())
+    } catch {
+      // engine not reachable yet; leave the previous list in place
+    }
+  }
+
+  useEffect(() => {
+    loadParams()
+  }, [])
+
   function selectPreset(id: TimeRangePresetId) {
     setActivePreset(id)
     setRange(computePresetRange(id))
@@ -108,7 +131,8 @@ export default function App() {
             activePreset={activePreset}
             onSelectPreset={selectPreset}
           />
-          <LogPreview logs={logs} />
+          <LogPreview logs={logs} params={params} />
+          <Params params={params} onDeclared={loadParams} />
           <Metrics range={range} />
           <Dashboards range={range} />
           <Alerts />