feat(settings): enhance settings handling and add startup messages, fixes #593 and fixes #594

- Add standalone mode detection via LORA_MANAGER_STANDALONE environment variable
- Improve error handling for settings file loading with specific JSON decode errors
- Add startup messages system to communicate configuration warnings and errors to users
- Include settings file path and startup messages in settings API response
- Automatically save settings when bootstrapping from defaults due to missing/invalid settings file
- Add configuration warnings collection for environment variables and other settings issues

The changes improve robustness of settings management and provide better user feedback when configuration issues occur.

tests/standalone/test_standalone_server.py

@@ -108,9 +108,10 @@ def test_validate_settings_warns_for_missing_model_paths(caplog, standalone_modu
         }
     )
 
-    assert standalone_module.validate_settings() is False
+    assert standalone_module.validate_settings() is True
     warning_lines = [record.message for record in caplog.records if record.levelname == "WARNING"]
-    assert any("CONFIGURATION WARNING" in line for line in warning_lines)
+    assert any("Standalone mode is using fallback" in line for line in warning_lines)
+    assert any("Model folders need setup" in line for line in warning_lines)
 
 
 def test_standalone_lora_manager_registers_routes(monkeypatch, tmp_path, standalone_module):