refactor(web): rename DayGroup/MonthGroup to TimelineDay/TimelineMonth

- Rename classes: DayGroup → TimelineDay, MonthGroup → TimelineMonth
- Use short variable names: dayGroup → day, monthGroup → month
- Update all method names and properties for consistency
- Convert relative imports to $lib alias convention

No functional changes.

web/src/lib/managers/timeline-manager/viewer-asset.svelte.ts

@@ -1,19 +1,18 @@
+import { calculateViewerAssetIntersecting } from '$lib/managers/timeline-manager/internal/intersection-support.svelte';
+import { TimelineDay } from '$lib/managers/timeline-manager/TimelineDay.svelte';
+import type { TimelineAsset } from '$lib/managers/timeline-manager/types';
 import type { CommonPosition } from '$lib/utils/layout-utils';
 
-import type { DayGroup } from './day-group.svelte';
-import { calculateViewerAssetIntersecting } from './internal/intersection-support.svelte';
-import type { TimelineAsset } from './types';
-
 export class ViewerAsset {
-  readonly #group: DayGroup;
+  readonly #day: TimelineDay;
 
   intersecting = $derived.by(() => {
     if (!this.position) {
       return false;
     }
 
-    const store = this.#group.monthGroup.timelineManager;
-    const positionTop = this.#group.absoluteDayGroupTop + this.position.top;
+    const store = this.#day.month.timelineManager;
+    const positionTop = this.#day.absoluteTop + this.position.top;
 
     return calculateViewerAssetIntersecting(store, positionTop, this.position.height);
   });
@@ -22,8 +21,8 @@ export class ViewerAsset {
   asset: TimelineAsset = <TimelineAsset>$state();
   id: string = $derived(this.asset.id);
 
-  constructor(group: DayGroup, asset: TimelineAsset) {
-    this.#group = group;
+  constructor(day: TimelineDay, asset: TimelineAsset) {
+    this.#day = day;
     this.asset = asset;
   }
 }