refactor(web): improve timeline scrubber labeling logic
Refactor the segment calculation in the timeline scrubber to improve code clarity and fix label positioning. Process months in reverse order for more intuitive label selection, use descriptive variable names, and remove unnecessary index tracking.
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
