fix(web): improve scrubber behavior on scroll-limited timelines (#22917)

Improves scroll indicator positioning when scrubbing through timelines with limited scrollable content (e.g., small albums). When a timeline's scrollable height is less than 50% of the viewport height, the scroll position is now properly distributed across the entire scrubber height, making the indicator more responsive and accurate.

Changes:
- Add `limitedScroll` state to detect scroll-constrained timelines (threshold: 50%)
- Introduce `ViewportTopMonth` type to handle lead-in/lead-out sections
- Calculate `totalViewerHeight` including top/bottom sections for accurate positioning
- Refactor scrubber to treat lead-in and lead-out as distinct scroll segments
- Update scroll position calculations to use relative percentages on constrained timelines

web/src/lib/managers/timeline-manager/timeline-manager.svelte.spec.ts

@@ -90,7 +90,7 @@ describe('TimelineManager', () => {
     });
 
     it('calculates timeline height', () => {
-      expect(timelineManager.timelineHeight).toBe(12_447.5);
+      expect(timelineManager.totalViewerHeight).toBe(12_507.5);
     });
   });