Initial commit: Shelly Manager with Textual CLI, Streamlit UI, and comprehensive .gitignore

Shelly device management app with mDNS/subnet discovery, inventory,
configuration, and mass operations for Gen1/Gen2+ devices.

Includes .gitignore excluding runtime data (device DB, user config),
AI conversation history, build artifacts, and common Python/OS patterns.

tests/test_import_order.py

@@ -0,0 +1,17 @@
+"""Guard against circular imports (e.g. api.client ↔ core.device_manager)."""
+
+from __future__ import annotations
+
+import importlib
+
+
+def test_mass_config_page_imports_cleanly() -> None:
+    """Importing the Mass Config page must not hit partially initialized api.client."""
+    importlib.import_module("shelly_manager.ui.pages.3_Mass_Config")
+
+
+def test_core_package_lazy_device_manager() -> None:
+    """DeviceManager is available via lazy attribute without eager import cycle."""
+    import shelly_manager.core as core
+
+    assert core.DeviceManager is not None