feat(dropzone): emit onUploadComplete callback with created count

Optional callback lets the parent route pop a post-upload banner without
lifting state into a store. Dashboard uses it to drive
UploadSuccessBanner (issue #296). Only fires when the server actually
created new documents — duplicates and errors do not trigger it.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/routes/DropZone.svelte

@@ -5,6 +5,12 @@ import { getErrorMessage } from '$lib/errors';
 
 const ACCEPTED_TYPES = ['application/pdf', 'image/jpeg', 'image/png', 'image/tiff'];
 
+interface Props {
+	onUploadComplete?: (count: number) => void;
+}
+
+let { onUploadComplete }: Props = $props();
+
 let isDragging = $state(false);
 let windowDragging = $state(false);
 let dragCounter = 0;
@@ -80,6 +86,7 @@ async function uploadFiles(files: File[]) {
 			const result = JSON.parse(body);
 			if (result.created?.length > 0) {
 				messages.push({ text: m.upload_success({ count: result.created.length }), isError: false });
+				onUploadComplete?.(result.created.length);
 			}
 			for (const doc of result.updated ?? []) {
 				messages.push({