Document GUI backend requirement and handle missing backend

docs/docs/content/01-getting-started/01-advanced_cli.md

@@ -116,6 +116,10 @@ Miscellaneous helper commands.
 | Verify script checksum | `util verify-checksum` | `seedpass util verify-checksum` |
 | Update script checksum | `util update-checksum` | `seedpass util update-checksum` |
 
+If you see a startup warning about a script checksum mismatch,
+run `seedpass util update-checksum` or choose "Generate Script Checksum"
+from the Settings menu to update the stored value.
+
 ### API Commands
 
 Run or stop the local HTTP API.

docs/docs/content/01-getting-started/06-gui_adapter.md

@@ -14,6 +14,11 @@ python -m seedpass_gui
 seedpass-gui
 ```
 
+Install a platform-specific BeeWare backend before launching the GUI. The
+distribution only bundles the headless `toga-dummy` backend for automated
+tests. Linux users should install `toga-gtk`, Windows users need
+`toga-winforms`, and macOS users require `toga-cocoa`.
+
 The GUI shares the same encrypted vault and configuration as the command line tool.
 
 To generate a packaged binary, run `briefcase build` (after the initial `briefcase create`).