perf(web): optimize shift+click range selection

- Add O(1) hasSelectionCandidate via #candidateSet (was O(n) .some() scan per thumbnail)
- Add assetId index lookup map in GalleryViewer for O(1) range slicing (was O(n) findIndex x2)
- Add same-month fast path in retrieveRange() to skip async iteration

web/src/lib/components/shared-components/gallery-viewer/GalleryViewer.svelte

@@ -85,6 +85,15 @@
   let shiftKeyIsDown = $state(false);
   let lastAssetMouseEvent: TimelineAsset | null = $state(null);
   let scrollTop = $state(0);
+
+  // Asset ID index lookup map for O(1) range computation
+  let assetIndexMap = $derived.by(() => {
+    const map = new Map<string, number>();
+    for (let i = 0; i < assets.length; i++) {
+      map.set(assets[i].id, i);
+    }
+    return map;
+  });
   let slidingWindow = $derived.by(() => {
     const top = (scrollTop || 0) - slidingWindowOffset;
     const bottom = top + viewport.height + slidingWindowOffset;
@@ -175,8 +184,12 @@
       return;
     }
 
-    let start = assets.findIndex((a) => a.id === startAsset.id);
-    let end = assets.findIndex((a) => a.id === endAsset.id);
+    let start = assetIndexMap.get(startAsset.id);
+    let end = assetIndexMap.get(endAsset.id);
+
+    if (start === undefined || end === undefined) {
+      return;
+    }
 
     if (start > end) {
       [start, end] = [end, start];

web/src/lib/managers/asset-multi-select-manager.svelte.ts

@@ -9,6 +9,7 @@ export type AssetMultiSelectOptions = {
 };
 export class AssetMultiSelectManager {
   #selectedMap = new SvelteMap<string, TimelineAsset>();
+  #candidateSet = new SvelteSet<string>();
 
   selectAll = $state(false);
   startAsset = $state<TimelineAsset | null>(null);
@@ -55,7 +56,7 @@ export class AssetMultiSelectManager {
   }
 
   hasSelectionCandidate(assetId: string) {
-    return this.candidates.some((asset) => asset.id === assetId);
+    return this.#candidateSet.has(assetId);
   }
 
   selectAsset(asset: TimelineAsset) {
@@ -64,7 +65,7 @@ export class AssetMultiSelectManager {
 
   selectAssets(assets: TimelineAsset[]) {
     for (const asset of assets) {
-      this.selectAsset(asset);
+      this.#selectedMap.set(asset.id, asset);
     }
   }
 
@@ -86,10 +87,15 @@ export class AssetMultiSelectManager {
 
   setAssetSelectionCandidates(assets: TimelineAsset[]) {
     this.candidates = assets;
+    this.#candidateSet.clear();
+    for (const asset of assets) {
+      this.#candidateSet.add(asset.id);
+    }
   }
 
   clearCandidates() {
     this.candidates = [];
+    this.#candidateSet.clear();
   }
 
   clear() {
@@ -101,6 +107,7 @@ export class AssetMultiSelectManager {
 
     // Range selection
     this.candidates = [];
+    this.#candidateSet.clear();
     this.startAsset = null;
   }
 }

web/src/lib/managers/timeline-manager/internal/search-support.svelte.ts

@@ -131,6 +131,24 @@ export async function retrieveRange(timelineManager: TimelineManager, start: Ass
     [startTimelineMonth, endTimelineMonth] = [endTimelineMonth, startTimelineMonth];
   }
 
+  // Fast path: both assets in the same loaded month — collect synchronously
+  if (startTimelineMonth === endTimelineMonth && startTimelineMonth.isLoaded) {
+    const range: TimelineAsset[] = [];
+    let collecting = false;
+    for (const asset of startTimelineMonth.assetsIterator()) {
+      if (!collecting && asset.id === startAsset.id) {
+        collecting = true;
+      }
+      if (collecting) {
+        range.push(asset);
+      }
+      if (asset.id === endAsset.id) {
+        break;
+      }
+    }
+    return range;
+  }
+
   const range: TimelineAsset[] = [];
   const startTimelineDay = startTimelineMonth.findTimelineDayForAsset(startAsset);
   for await (const targetAsset of timelineManager.assetsIterator({