fix(comments): open panel on annotation creation and enlarge comment count pill

- Auto-open AnnotationCommentPanel immediately after drawing a new annotation
- Move comment count pill to bottom-right corner (was centered at bottom)
- Increase pill size: font 11px bold, padding 2px 6px, min-width 20px, drop shadow

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

frontend/src/lib/components/PdfViewer.svelte

@@ -217,6 +217,7 @@ async function handleAnnotationDraw(rect: { x: number; y: number; width: number;
 		if (res.ok) {
 			const created: Annotation = await res.json();
 			annotations = [...annotations, created];
+			activeAnnotationId = created.id;
 		}
 	} catch {
 		// ignore