feat: add PDF upload support to Fliesstext-Import

The import page now accepts PDF files in addition to TXT.
Backend uses pdf-parse to extract text from uploaded PDFs
before sending to AI for paragraph parsing.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

next.config.ts

@@ -2,6 +2,7 @@ import type { NextConfig } from "next";
 
 const nextConfig: NextConfig = {
   output: "standalone",
+  serverExternalPackages: ["pdf-parse"],
 };
 
 export default nextConfig;