Add Metrics UI: declare metrics and view derived bucket values (ticket 012)

Replaces the Metrics ComingSoon placeholder with a form for declaring
metrics (name, message_match_regex, param matchers) and a list/detail
view of declared metrics whose bucket values refetch on selection or
active-range change (no continuous polling, since declare's backfill
already resolves synchronously with the POST).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

webapp/src/App.tsx

@@ -4,6 +4,7 @@ import { FilterBar } from './components/FilterBar'
 import { LogTimeline, type TimeRange } from './components/LogTimeline'
 import { LogPreview } from './components/LogPreview'
 import { ComingSoon } from './components/ComingSoon'
+import { Metrics } from './components/Metrics'
 import { fetchLogs, fetchStats, fetchTimeline, type LogEntry, type Stats, type TimelineBucket } from './lib/api'
 
 const POLL_MS = 2000
@@ -52,7 +53,7 @@ export default function App() {
         <FilterBar value={filter} onChange={setFilter} />
         <LogTimeline buckets={buckets} range={range} onSelectRange={setRange} />
         <LogPreview logs={visibleLogs} />
-        <ComingSoon title="Metrics" />
+        <Metrics range={range} />
         <ComingSoon title="Dashboards" />
       </div>
     </div>