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/utils/timeline-util.ts

@@ -1,4 +1,4 @@
-import type { TimelineAsset } from '$lib/managers/timeline-manager/types';
+import type { TimelineAsset, ViewportTopMonth } from '$lib/managers/timeline-manager/types';
 import { locale } from '$lib/stores/preferences.store';
 import { getAssetRatio } from '$lib/utils/asset-utils';
 import { AssetTypeEnum, type AssetResponseDto } from '@immich/sdk';
@@ -24,7 +24,7 @@ export type TimelineDateTime = TimelineDate & {
 };
 
 export type ScrubberListener = (scrubberData: {
-  scrubberMonth: { year: number; month: number };
+  scrubberMonth: ViewportTopMonth;
   overallScrollPercent: number;
   scrubberMonthScrollPercent: number;
 }) => void | Promise<void>;