fix: Correct LV vs VG selection logic

The refresh_drives method was incorrectly showing VGs for all modes.
Now correctly shows:
- LVs for: lv_to_lv, lv_to_raw, lv_to_borg, files_to_borg
- VGs for: vg_to_raw, vg_to_borg

This fixes the issue where users couldn't select individual LVs.

simple_backup_gui.py

@@ -269,7 +269,7 @@ class SimpleBackupGUI:
             self.source_combo.grid(row=0, column=0, sticky=(tk.W, tk.E))
         
         # Source options
-        if mode == "vg_to_raw":
+        if mode == "vg_to_raw" or mode == "vg_to_borg":
             # Show volume groups
             success, output = self.run_command("vgs --noheadings -o vg_name,vg_size,pv_count", show_output=False)
             if success:
@@ -289,7 +289,7 @@ class SimpleBackupGUI:
                 self.log("No volume groups found")
                 self.source_combo['values'] = []
         else:
-            # Show logical volumes (all LV modes)
+            # Show logical volumes (all LV modes: lv_to_lv, lv_to_raw, lv_to_borg, files_to_borg)
             success, output = self.run_command("lvs --noheadings -o lv_path,lv_size,vg_name", show_output=False)
             if success:
                 lv_list = []