refactor(web): move timeline-manager ops back into month-group; clean up API

Consolidates asset operation logic within TimelineManager class and removes the now redundant 
operations-support.svelte.ts file. 

Combines addAsset/updateAsset to be upsertAsset. 


Changes:
- Move `addAssetsToMonthGroups` logic into TimelineManager's `addAssetsToSegments`, `upsertAssetIntoSegment`, `postCreateSegments`, and `postUpsert` methods
- Move `runAssetOperation` from operations-support into TimelineManager's private `#runAssetOperation` method
- Rename public `addAssets`/`updateAssets` methods to unified `upsertAssets` for consistency
- Delete internal/operations-support.svelte.ts
- Update WebsocketSupport to use `upsertAssets` for both add and update operations
- Fix AssetOperation return type to allow undefined/void operations (not just `{ remove: boolean }`)
- Update MonthGroup constructor to accept `loaded` parameter for better initialization control
- Update all test references from `addAssets`/`updateAssets` to `upsertAssets`

This refactoring improves code maintainability by eliminating duplicate logic and consolidating all asset operations within the TimelineManager class where they belong.

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

@@ -110,13 +110,9 @@
       case AssetAction.ARCHIVE:
       case AssetAction.UNARCHIVE:
       case AssetAction.FAVORITE:
-      case AssetAction.UNFAVORITE: {
-        timelineManager.updateAssets([action.asset]);
-        break;
-      }
-
+      case AssetAction.UNFAVORITE:
       case AssetAction.ADD: {
-        timelineManager.addAssets([action.asset]);
+        timelineManager.upsertAssets([action.asset]);
         break;
       }
 
@@ -135,7 +131,7 @@
         break;
       }
       case AssetAction.REMOVE_ASSET_FROM_STACK: {
-        timelineManager.addAssets([toTimelineAsset(action.asset)]);
+        timelineManager.upsertAssets([toTimelineAsset(action.asset)]);
         if (action.stack) {
           //Have to unstack then restack assets in timeline in order to update the stack count in the timeline.
           updateUnstackedAssetInTimeline(