Improve config flow UX for multi-shade setups

Reuse the home key from already-configured shades so adding subsequent
shades skips the key step. Show human-readable shade names from the hub
in the device picker. Allow selecting multiple shades at once instead of
repeating the flow for each one. Default to hub fetch as the key method.

custom_components/hunterdouglas_powerview_ble/api.py

@@ -134,6 +134,11 @@ class PowerViewBLE:
     def encrypted(self, value: bool) -> None:
         self._is_encrypted = value
 
+    @property
+    def has_key(self) -> bool:
+        """Return True if a valid homekey was provided."""
+        return self._cipher is not None
+
     @property
     def info(self) -> PVDeviceInfo:
         """Return device information, e.g. SW version."""