fix(a11y): bump progress bar text to text-xs minimum, add motion-safe to upload animation

- text-[9px]/text-[10px] in required-fields bar raised to text-xs (12px),
  meeting the project minimum for the 60+ audience (WCAG 1.4.4)
- Upload animation now uses motion-safe: prefix so it stops for users
  with prefers-reduced-motion set (WCAG 2.1 SC 2.3.3)
- Strengthened UploadZone tests: onCancel uses [role=status] button
  selector instead of first-button heuristic; added positive file
  selection test (valid PDF calls onFile), file-too-large test, and
  MIME rejection now also asserts the error message is visible

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

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

@@ -55,7 +55,9 @@ function handleDrop(e: DragEvent) {
 	{#if isUploading}
 		<div role="status" class="flex flex-col items-center gap-3 text-center">
 			<div class="h-0.5 w-48 overflow-hidden rounded-full bg-white/10">
-				<div class="h-full animate-[slide_1.4s_ease-in-out_infinite] bg-brand-mint/70"></div>
+				<div
+					class="h-full bg-brand-mint/70 motion-safe:animate-[slide_1.4s_ease-in-out_infinite]"
+				></div>
 			</div>
 			<p class="max-w-[200px] truncate text-xs font-medium text-brand-mint/70">{filename}</p>
 			<p class="text-xs text-white/40">Wird hochgeladen …</p>