fix(annotations): show annotations when entering annotate mode + restore documentFileHash

- PdfViewer: add $effect that forces showAnnotations=true when annotateMode
  becomes true, so hiding annotations before drawing no longer breaks drawing
- DocumentViewer: restore missing fileHash field on Doc type and pass
  documentFileHash to PdfViewer (lost when rebase dropped the merge commit)

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

frontend/src/lib/components/PdfViewer.svelte

@@ -277,6 +277,10 @@ $effect(() => {
 	}
 });
 
+$effect(() => {
+	if (annotateMode) showAnnotations = true;
+});
+
 function prevPage() {
 	if (currentPage > 1) currentPage -= 1;
 }