Raise file upload limit to 50 MB

Was 10 MB — blocked zip files at or near that size. Supabase storage
supports up to 50 MB; mime-type restrictions were already removed.

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

src/components/FileAttachment.jsx

@@ -1,7 +1,7 @@
 import { useState, useRef } from 'react';
 
 const MAX_FILES = 20;
-const MAX_SIZE_MB = 10;
+const MAX_SIZE_MB = 50;
 const MAX_SIZE_BYTES = MAX_SIZE_MB * 1024 * 1024;
 
 const formatSize = (bytes) => {