fix: add missing _AVD_SAMSUNG_HW_DEVICE variable definition

Fixes NameError: name '_AVD_SAMSUNG_HW_DEVICE' is not defined
when creating Samsung devices. Set to 'pixel_6' as fallback.

This variable is used when creating AVD for Samsung fingerprints,
matching the hvdmanager --device parameter.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Author: code-root

backend/core/emulator/avd.py

@@ -29,6 +29,9 @@
     "SM-N986U": "Samsung Galaxy Note 20 Ultra",
 }
 
+# Samsung AVD hardware profile — fallback to pixel_6 if not available
+_AVD_SAMSUNG_HW_DEVICE = "pixel_6"
+
 
 def _is_samsung_fingerprint_hw(
     manufacturer: Optional[str], brand: Optional[str], device_model: Optional[str]