Move TimeRange types to lib/timeRange to break import cycle

CollapsibleSection.tsx and LogTimeline.tsx imported from each other
(TimeRange/TimeRangePresetId one way, sectionHeaderClassName the
other). Move the range types/helpers into a dependency-free
webapp/src/lib/timeRange.ts and repoint all importers there.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TeyhRfMYjMditek6geDRoQ

webapp/src/components/PanelChart.tsx

@@ -14,7 +14,7 @@ import {
   YAxis,
 } from 'recharts'
 import type { ChartType, PromqlSeries } from '../lib/api'
-import type { TimeRange } from './LogTimeline'
+import type { TimeRange } from '../lib/timeRange'
 
 /// Renders a label combination as a series key/legend string, e.g. `{"path":"/a"}` -> `path=/a`; no labels renders as `value`.
 export function seriesLabel(labels: Record<string, string>): string {