fix(bulk-upload): populate aria-live region with active file title

The sr-only aria-live div was always empty, so screen readers never
announced file switches. Derived activeAnnouncement from the active
entry and bound it to the div's text content.

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

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

@@ -24,6 +24,8 @@ let {
 let trackEl = $state<HTMLDivElement | null>(null);
 let listEl = $state<HTMLUListElement | null>(null);
 
+const activeAnnouncement = $derived(files.find((f) => f.id === activeId)?.title ?? '');
+
 function scrollPrev() {
 	trackEl?.scrollBy({ left: -120, behavior: 'smooth' });
 }
@@ -58,7 +60,7 @@ $effect(() => {
 });
 </script>
 
-<div aria-live="polite" aria-atomic="true" class="sr-only"></div>
+<div aria-live="polite" aria-atomic="true" class="sr-only">{activeAnnouncement}</div>
 <div
 	data-testid="file-switcher-strip"
 	class="flex h-11 shrink-0 items-center gap-1 border-t border-line bg-pdf-ctrl px-2"