docs: explain BeeWare adapter

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

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+# BeeWare GUI Adapter
+
+SeedPass ships with a proof-of-concept graphical interface built using [BeeWare](https://beeware.org). The GUI interacts with the same core services as the CLI by instantiating wrappers around `PasswordManager`.
+
+## VaultService and EntryService
+
+`VaultService` provides thread-safe access to vault operations like exporting, importing, unlocking and locking the vault. `EntryService` exposes methods for listing, searching and modifying entries. Both classes live in `seedpass.core.api` and hold a `PasswordManager` instance protected by a `threading.Lock` to ensure safe concurrent access.
+
+```python
+class VaultService:
+    """Thread-safe wrapper around vault operations."""
+    def __init__(self, manager: PasswordManager) -> None:
+        self._manager = manager
+        self._lock = Lock()
+```
+
+```python
+class EntryService:
+    """Thread-safe wrapper around entry operations."""
+    def __init__(self, manager: PasswordManager) -> None:
+        self._manager = manager
+        self._lock = Lock()
+```
+
+## BeeWare Windows
+
+The GUI defines two main windows in `src/seedpass_gui/app.py`. `LockScreenWindow` prompts for the master password and then opens `MainWindow` to display the vault entries.
+
+```python
+class LockScreenWindow(toga.Window):
+    """Window prompting for the master password."""
+    def __init__(self, app: SeedPassApp, vault: VaultService, entries: EntryService) -> None:
+        super().__init__("Unlock Vault")
+        self.app = app
+        self.vault = vault
+        self.entries = entries
+        ...
+```
+
+```python
+class MainWindow(toga.Window):
+    """Main application window showing vault entries."""
+    def __init__(self, app: SeedPassApp, vault: VaultService, entries: EntryService) -> None:
+        super().__init__("SeedPass")
+        self.app = app
+        self.vault = vault
+        self.entries = entries
+        ...
+```
+
+Each window receives the service instances and calls methods such as `vault.unlock()` or `entries.add_entry()` when buttons are pressed. This keeps the UI thin while reusing the core logic.
+
+## Asynchronous Synchronization
+
+`PasswordManager` performs network synchronization with Nostr using `asyncio`. Methods like `start_background_vault_sync()` create a coroutine that calls `sync_vault_async()` in a background thread or task without blocking the UI.
+
+```python
+async def sync_vault_async(self, alt_summary: str | None = None) -> dict[str, list[str] | str] | None:
+    """Publish the current vault contents to Nostr and return event IDs."""
+    ...
+```
+
+```python
+def start_background_vault_sync(self, alt_summary: str | None = None) -> None:
+    if getattr(self, "offline_mode", False):
+        return
+    def _worker() -> None:
+        asyncio.run(self.sync_vault_async(alt_summary=alt_summary))
+    try:
+        loop = asyncio.get_running_loop()
+    except RuntimeError:
+        threading.Thread(target=_worker, daemon=True).start()
+    else:
+        asyncio.create_task(self.sync_vault_async(alt_summary=alt_summary))
+```
+
+This approach ensures synchronization happens asynchronously whether the GUI is running inside or outside an existing event loop.