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

iridium_installer/ui/pages/partitioning.py

@@ -558,13 +558,27 @@ class PartitioningPage(Adw.Bin):
                 error_label.set_text("Can't set partition name to Linux filesystem")
                 return
 
-            size_mb = int(size_entry.get_text())
+            size_text = size_entry.get_text()
+            try:
+                size_mb = int(size_text)
+                # If they are using the max proposed size, use 0 to avoid alignment issues
+                # with sgdisk +sizeM logic exceeding disk boundaries.
+                max_mb = int(data["bytes"] / (1024*1024))
+                if size_mb >= max_mb:
+                    size_mb = 0
+            except ValueError:
+                error_label.set_text("Invalid size value")
+                return
+
             type_code = types[selected_type]
             fstype = fs_options[fs_dropdown.get_selected()]
             
+            try:
                 create_partition(self.current_disk_path, size_mb, type_code, name, fstype)
                 win.close()
                 self.load_partitions(self.current_disk_path)
+            except Exception as e:
+                error_label.set_text(f"Error: {str(e)}")
 
         btn = Gtk.Button(label="Create")
         btn.add_css_class("suggested-action")