refactor(web): extract common timeline functionality into PhotostreamManager base classes

Create abstract PhotostreamManager and PhotostreamSegment base classes to enable reusable      
timeline-like components. This refactoring extracts common viewport management, scroll         
handling, and segment operations from TimelineManager and MonthGroup into reusable             
abstractions.                                                                                  
                                                                                                  
Changes:                                                                                       
 - Add PhotostreamManager.svelte.ts with viewport and scroll management                         
 - Add PhotostreamSegment.svelte.ts with segment positioning and intersection logic             
 - Refactor TimelineManager to extend PhotostreamManager                                        
 - Refactor MonthGroup to extend PhotostreamSegment                                             
 - Add utility functions for segment identification and date formatting                         
 - Update tests to reflect new inheritance structure

web/src/lib/components/timeline/Timeline.svelte

@@ -19,7 +19,12 @@
   import { isSelectingAllAssets } from '$lib/stores/assets-store.svelte';
   import { mobileDevice } from '$lib/stores/mobile-device.svelte';
   import { navigate } from '$lib/utils/navigation';
-  import { getTimes, type ScrubberListener, type TimelineYearMonth } from '$lib/utils/timeline-util';
+  import {
+    getSegmentIdentifier,
+    getTimes,
+    type ScrubberListener,
+    type TimelineYearMonth,
+  } from '$lib/utils/timeline-util';
   import { type AlbumResponseDto, type PersonResponseDto } from '@immich/sdk';
   import { DateTime } from 'luxon';
   import { onMount, type Snippet } from 'svelte';
@@ -478,7 +483,7 @@
           break;
         }
         if (started) {
-          await timelineManager.loadMonthGroup(monthGroup.yearMonth);
+          await timelineManager.loadSegment(monthGroup.identifier);
           for (const asset of monthGroup.assetsIterator()) {
             if (deselect) {
               assetInteraction.removeAssetFromMultiselectGroup(asset.id);
@@ -553,7 +558,7 @@
   $effect(() => {
     if ($showAssetViewer) {
       const { localDateTime } = getTimes($viewingAsset.fileCreatedAt, DateTime.local().offset / 60);
-      void timelineManager.loadMonthGroup({ year: localDateTime.year, month: localDateTime.month });
+      void timelineManager.loadSegment(getSegmentIdentifier({ year: localDateTime.year, month: localDateTime.month }));
     }
   });
 </script>