fix(bulk-upload): add gradient overflow indicators to chip strip

Adds pointer-events-none left/right gradient fade overlays on the
FileSwitcherStrip track div so mouse-only users can see when more
chips are hidden beyond the visible area. The scrollbar is hidden
(scrollbar-width:none) so gradients are the only overflow signal.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/components/document/FileSwitcherStrip.svelte

@@ -83,6 +83,14 @@ $effect(() => {
 		>‹</button
 	>
 
+	<!-- Gradient fade overlays signal hidden overflow to pointer-only users -->
+	<div class="relative flex flex-1 overflow-hidden">
+		<div
+			class="pointer-events-none absolute inset-y-0 left-0 z-10 w-6 bg-gradient-to-r from-pdf-ctrl to-transparent"
+		></div>
+		<div
+			class="pointer-events-none absolute inset-y-0 right-0 z-10 w-6 bg-gradient-to-l from-pdf-ctrl to-transparent"
+		></div>
 		<div bind:this={trackEl} class="flex flex-1 gap-1 overflow-x-auto" style="scrollbar-width:none">
 			<ul bind:this={listEl} role="list" class="flex flex-row gap-1 py-1">
 				{#each files as entry, i (entry.id)}
@@ -130,6 +138,7 @@ $effect(() => {
 				{/each}
 			</ul>
 		</div>
+	</div>
 
 	<button
 		type="button"