fix(settings): catch PermissionError in load_settings + error-path tests (#1570)

PermissionError was not in the except tuple so an unreadable settings.json
would crash the app instead of falling back to defaults. Added alongside the
existing FileNotFoundError/JSONDecodeError/ValueError catches.

Also adds test_settings_error_paths.py covering all four failure modes:
missing file, corrupted JSON, wrong type, and permission denied.

src/settings.py

@@ -187,7 +187,7 @@ def load_settings() -> dict:
         if not isinstance(saved, dict):
             raise ValueError("settings must be an object")
         merged = {**DEFAULT_SETTINGS, **saved}
-    except (FileNotFoundError, json.JSONDecodeError, ValueError):
+    except (FileNotFoundError, PermissionError, json.JSONDecodeError, ValueError):
         merged = dict(DEFAULT_SETTINGS)
     _settings_cache = (now, merged)
     return merged