feat(frontend): add thumbnailUrl helper with cache-bust param

Pure function returning /api/documents/{id}/thumbnail?v=<timestamp>
or null when thumbnailKey is missing. The encoded timestamp changes
whenever the backend regenerates a thumbnail (file replace),
invalidating browser caches despite the immutable Cache-Control.

Refs #307

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/thumbnails.ts

@@ -0,0 +1,18 @@
+type ThumbnailDoc = {
+	id: string;
+	thumbnailKey?: string;
+	thumbnailGeneratedAt?: string;
+};
+
+/**
+ * Builds the URL for a document thumbnail image, or returns null when the document
+ * has no thumbnail yet. When `thumbnailGeneratedAt` is present it is appended as a
+ * `?v=…` query param so the browser / proxy cache is invalidated whenever the file
+ * is replaced (the backend regenerates thumbnails at the same S3 key on replace).
+ */
+export function thumbnailUrl(doc: ThumbnailDoc): string | null {
+	if (!doc.thumbnailKey) return null;
+	const base = `/api/documents/${doc.id}/thumbnail`;
+	if (!doc.thumbnailGeneratedAt) return base;
+	return `${base}?v=${encodeURIComponent(doc.thumbnailGeneratedAt)}`;
+}