feat: Extract common StreamWithViewer component

- Create new StreamWithViewer component to handle asset viewer lifecycle
  and navigation
  - Move beforeNavigate/afterNavigate hooks from Timeline to StreamWithViewer
  - Extract asset viewer Portal rendering and close handler to wrapper
  component
  - Move timeline segment loading logic for viewed assets to StreamWithViewer
  - Simplify Timeline component by removing ~76 lines of navigation/viewer
  code
  - Remove showSkeleton state management from Timeline (now handled by
  PhotostreamWithScrubber)

  This separation of concerns makes the Timeline component more focused on
  rendering while StreamWithViewer handles all viewer-related navigation and state
  management.The new component can be reused by other photostream-like components that
  need asset viewer functionality.

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

@@ -0,0 +1,76 @@
+<script lang="ts">
+  import { afterNavigate, beforeNavigate } from '$app/navigation';
+  import { page } from '$app/state';
+  import Portal from '$lib/elements/Portal.svelte';
+  import type { PhotostreamManager } from '$lib/managers/photostream-manager/PhotostreamManager.svelte';
+  import { assetViewingStore } from '$lib/stores/asset-viewing.store';
+  import { isAssetViewerRoute, navigate } from '$lib/utils/navigation';
+  import { getSegmentIdentifier, getTimes } from '$lib/utils/timeline-util';
+  import { DateTime } from 'luxon';
+  import { type Snippet } from 'svelte';
+
+  interface Props {
+    timelineManager: PhotostreamManager;
+    children?: Snippet;
+    assetViewer: Snippet<[{ onViewerClose: (asset: { id: string }) => Promise<void> }]>;
+    onAfterNavigateComplete: (args: { scrollToAssetQueryParam: boolean }) => void;
+  }
+
+  let { timelineManager, children, assetViewer, onAfterNavigateComplete }: Props = $props();
+
+  let { isViewing: showAssetViewer, asset: viewingAsset, gridScrollTarget } = assetViewingStore;
+
+  // tri-state boolean
+  let initialLoadWasAssetViewer: boolean | null = null;
+  let hasNavigatedToOrFromAssetViewer: boolean = false;
+  let timelineScrollPositionInitialized = false;
+
+  beforeNavigate(({ from, to }) => {
+    timelineManager.suspendTransitions = true;
+    hasNavigatedToOrFromAssetViewer = isAssetViewerRoute(to) || isAssetViewerRoute(from);
+  });
+
+  const completeAfterNavigate = () => {
+    const assetViewerPage = !!(page.route.id?.endsWith('/[[assetId=id]]') && page.params.assetId);
+    let isInitial = false;
+    // Set initial load state only once
+    if (initialLoadWasAssetViewer === null) {
+      initialLoadWasAssetViewer = assetViewerPage && !hasNavigatedToOrFromAssetViewer;
+      isInitial = true;
+    }
+
+    let scrollToAssetQueryParam = false;
+    if (
+      !timelineScrollPositionInitialized &&
+      ((isInitial && !assetViewerPage) || // Direct timeline load
+        (!isInitial && hasNavigatedToOrFromAssetViewer)) // Navigated from asset viewer
+    ) {
+      scrollToAssetQueryParam = true;
+      timelineScrollPositionInitialized = true;
+    }
+
+    return onAfterNavigateComplete({ scrollToAssetQueryParam });
+  };
+  afterNavigate(({ complete }) => void complete.then(completeAfterNavigate, completeAfterNavigate));
+
+  const onViewerClose = async (asset: { id: string }) => {
+    assetViewingStore.showAssetViewer(false);
+    $gridScrollTarget = { at: asset.id };
+    await navigate({ targetRoute: 'current', assetId: null, assetGridRouteSearchParams: $gridScrollTarget });
+  };
+
+  $effect(() => {
+    if ($showAssetViewer) {
+      const { localDateTime } = getTimes($viewingAsset.fileCreatedAt, DateTime.local().offset / 60);
+      void timelineManager.loadSegment(getSegmentIdentifier({ year: localDateTime.year, month: localDateTime.month }));
+    }
+  });
+</script>
+
+{@render children?.()}
+
+<Portal target="body">
+  {#if $showAssetViewer}
+    {@render assetViewer({ onViewerClose })}
+  {/if}
+</Portal>