feat(ui): Add warning for unmounted encrypted drives

Adds a warning message to the main UI.

The message "Accurate detection is only possible when the drive is mounted" is now displayed in addition to the standard "You can start a backup here" text, under the following conditions:
- The backup mode is active.
- The "Encrypt Backup" option is enabled for the current profile.
- The corresponding encrypted drive is not currently mounted.

This provides clearer feedback to the user on why size calculations might be unavailable or inaccurate for encrypted targets.

core/pbp_app_config.py

@@ -229,6 +229,7 @@ class Msg:
         "ready_for_first_backup": _("Everything is ready for your first backup."),
         "backup_mode": _("Backup Mode"),
         "backup_mode_info": _("Backup Mode: You can start a backup here."),
+        "encrypted_unmounted_warning": _("Accurate detection is only possible when the drive is mounted."),
         "restore_mode_info": _("Restore Mode: You can start a restore here."),
         "advanced_settings_title": _("Advanced Settings"),
         "animation_settings_title": _("Animation Settings"),

pyimage_ui/drawing.py

@@ -290,7 +290,21 @@ class Drawing:
             elif self.app.is_first_backup:
                 info_messages.append(Msg.STR["ready_for_first_backup"])
             elif self.app.mode == "backup":
+                # Base message is always shown after calculation in backup mode
                 info_messages.append(Msg.STR["backup_mode_info"])
+
+                # Additionally, show a warning if encryption is on but the drive is not mounted
+                if self.app.encrypted_var.get():
+                    dest_path = self.app.destination_path
+                    profile_name = self.app.left_canvas_data.get('folder')
+
+                    is_mounted = False
+                    if dest_path and profile_name:
+                        is_mounted = self.app.backup_manager.encryption_manager.is_mounted(
+                            dest_path, profile_name)
+
+                    if not is_mounted:
+                        info_messages.append(Msg.STR["encrypted_unmounted_warning"])
             else:
                 info_messages.append(Msg.STR["restore_mode_info"])