fix(partitioning): avoid alignment errors by using fill-gap logic for full space

iridium_installer/backend/disk.py

@@ -120,10 +120,13 @@ def create_partition(disk_device, size_mb, type_code="8300", name="Linux filesys
         next_num += 1
 
     # Use -g (mbrtogpt) to ensure we can work on the disk if it was MBR
+    # size_mb=0 means use all available space in the gap
+    size_spec = f"+{size_mb}M" if size_mb > 0 else "0"
+    
     run_command([
         "sgdisk",
         "-g",
-        "-n", f"{next_num}:0:+{size_mb}M", 
+        "-n", f"{next_num}:0:{size_spec}", 
         "-t", f"{next_num}:{type_code}", 
         "-c", f"{next_num}:{name}", 
         disk_device