Fix P2S empty archive tiles - add root folder to FTP search paths (Issue #146)

Printers without an SD card store files in the root folder `/` instead of
`/cache`. Added root folder to search paths in both main.py and bambu_ftp.py
so 3MF files can be found regardless of storage configuration.

Closes #146

backend/app/main.py

@@ -720,7 +720,7 @@ async def on_print_start(printer_id: int, data: dict):
         if not downloaded_filename and (filename or subtask_name):
             search_term = (subtask_name or filename).lower().replace(".gcode", "").replace(".3mf", "")
             logger.info(f"Direct FTP download failed, searching directories for '{search_term}'")
-            search_dirs = ["/cache", "/model", "/data", "/data/Metadata"]
+            search_dirs = ["/cache", "/model", "/data", "/data/Metadata", "/"]
             for search_dir in search_dirs:
                 if downloaded_filename:
                     break

backend/app/services/bambu_ftp.py

@@ -323,7 +323,7 @@ class BambuFTPClient:
         # Calculate used space by listing root directories
         try:
             total_used = 0
-            dirs_to_scan = ["/cache", "/timelapse", "/model", "/data", "/data/Metadata"]
+            dirs_to_scan = ["/cache", "/timelapse", "/model", "/data", "/data/Metadata", "/"]
 
             for dir_path in dirs_to_scan:
                 try: