feat: make GUI dependencies optional

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

@@ -14,10 +14,17 @@ python -m seedpass_gui
 seedpass-gui
 ```
 
-Only `toga-core` and the headless `toga-dummy` backend ship with the project.
-The installation scripts automatically install the correct BeeWare backend so
-the GUI works out of the box. If you set up SeedPass manually, install the
-backend for your platform:
+GUI dependencies are optional. Install them alongside SeedPass with:
+
+```bash
+pip install "seedpass[gui]"
+
+# or when working from a local checkout
+pip install -e .[gui]
+```
+
+After installing the optional GUI extras, add the BeeWare backend for your
+platform:
 
 ```bash
 # Linux