fix: move upload endpoint to root /upload route (works from both app and atelier)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend-v2/src/routes/upload/+server.ts

@@ -0,0 +1,25 @@
+import { json } from '@sveltejs/kit';
+import type { RequestHandler } from './$types';
+import fs from 'fs';
+import path from 'path';
+
+const UPLOAD_DIR = '/app/screenshots';
+
+export const POST: RequestHandler = async ({ request }) => {
+    const formData = await request.formData();
+    const file = formData.get('file') as File;
+    if (!file) return json({ error: 'No file' }, { status: 400 });
+
+    const timestamp = new Date().toISOString().replace(/[:.]/g, '-').slice(0, 19);
+    const ext = file.name.split('.').pop() || 'png';
+    const filename = `${timestamp}.${ext}`;
+
+    const buffer = Buffer.from(await file.arrayBuffer());
+
+    // Ensure directory exists
+    if (!fs.existsSync(UPLOAD_DIR)) fs.mkdirSync(UPLOAD_DIR, { recursive: true });
+
+    fs.writeFileSync(path.join(UPLOAD_DIR, filename), buffer);
+
+    return json({ ok: true, filename, path: `/app/screenshots/${filename}` });
+};