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/CollapsibleSection.tsx

@@ -1,5 +1,5 @@
 import { useState } from 'react'
-import type { TimeRange, TimeRangePresetId } from './LogTimeline'
+import type { TimeRange, TimeRangePresetId } from '../lib/timeRange'
 
 /// Shared look for a primary section's title bar: shaded so it reads as a
 /// distinct band rather than blending into the section body below it.