Refetch dashboard panels on the shared polling cadence

PanelPreview only refetched its PromQL series when panel.promql,
chart_type, or range bounds changed, so panels went stale under a
fixed range or between rolling-preset ticks that didn't move the
window. Add a POLL_MS-driven tick folded into the same fetch effect
(alongside the existing deps) so it refreshes on the same cadence as
App.tsx's log/timeline polling without double-fetching when a range
change and a tick coincide. POLL_MS is now shared via
webapp/src/lib/polling.ts instead of living only in App.tsx.

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

webapp/src/App.tsx

@@ -19,8 +19,7 @@ import {
   type TimelineBucket,
 } from './lib/api'
 import { DocsNavProvider } from './lib/docsNav'
-
-const POLL_MS = 2000
+import { POLL_MS } from './lib/polling'
 
 /// Reads the routing params `PopOutButton` writes: `section` picks which primary section to render full-page, and `startMs`/`endMs` carry over the range for sections with no `LogTimeline` of their own to pick one.
 function parsePopOutQuery(): { section: PopOutSectionId | null; range: TimeRange | null } {