Extract asset grid core logic into asset-grid-without-scrubber component

• Move all timeline rendering and scrolling logic to new component
• Keep scrubber logic and interaction handlers in original asset-grid 
• Use composition pattern with header snippet for scrubber integration
• Simplify asset-grid to ~450 lines by extracting ~390 lines of core logic

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

@@ -42,6 +42,7 @@ export class TimelineManager {
   isInitialized = $state(false);
   months: MonthGroup[] = $state([]);
   topSectionHeight = $state(0);
+  bottomSectionHeight = $state(60);
   timelineHeight = $derived(this.months.reduce((accumulator, b) => accumulator + b.height, 0) + this.topSectionHeight);
   assetCount = $derived(this.months.reduce((accumulator, b) => accumulator + b.assetsCount, 0));
 
@@ -557,4 +558,13 @@ export class TimelineManager {
   getAssetOrder() {
     return this.#options.order ?? AssetOrder.Desc;
   }
+
+  getMaxScrollPercent() {
+    const totalHeight = this.timelineHeight + this.bottomSectionHeight + this.topSectionHeight;
+    return (totalHeight - this.viewportHeight) / totalHeight;
+  }
+
+  getMaxScroll() {
+    return this.topSectionHeight + this.bottomSectionHeight + (this.timelineHeight - this.viewportHeight);
+  }
 }