fix(backend): improve partition creation robustness and formatting

iridium_installer/backend/disk.py

@@ -119,18 +119,24 @@ def create_partition(disk_device, size_mb, type_code="8300", name="Linux filesys
     while next_num in existing_nums:
         next_num += 1
 
+    # Use -g (mbrtogpt) to ensure we can work on the disk if it was MBR
     run_command([
-        "sgdisk", 
+        "sgdisk",
+        "-g",
         "-n", f"{next_num}:0:+{size_mb}M", 
         "-t", f"{next_num}:{type_code}", 
         "-c", f"{next_num}:{name}", 
         disk_device
     ])
+    
     run_command(["partprobe", disk_device])
     
-    # Wait for partition node
-    import time
-    time.sleep(1)
+    # Wait for partition node to appear
+    try:
+        run_command(["udevadm", "settle", "--timeout=5"])
+    except Exception:
+        import time
+        time.sleep(2)
     
     part_dev = get_partition_device(disk_device, next_num)