feat(settings): add an explicit opt-out from persisted portable mode

Setting LORA_MANAGER_PORTABLE=1 once wrote use_portable_settings: true
into the plugin's own settings.json, and every later run of every
instance sharing that plugin folder then read and wrote the portable
settings directory. There was no way back except editing the file by
hand, which is exactly the trap a user hit while following the FAQ's
instructions for isolating a second instance (#1114).

LORA_MANAGER_PORTABLE=0 is now the explicit exit:

- _should_use_portable_settings honours "0" as a forced off, so the
  resolved settings directory no longer depends on the persisted flag.
- SettingsManager clears the persisted flag in that case, so later runs
  without the variable stay on the shared settings directory.

Unset or unrecognised values keep the previous behaviour: the persisted
flag decides, so existing portable installs are unaffected.
LORA_MANAGER_SETTINGS_DIR still takes precedence over both.

py/services/settings_manager.py

@@ -37,6 +37,7 @@ from ..utils.constants import (
 from ..utils.preview_selection import VALID_MATURE_BLUR_LEVELS
 from ..utils.settings_paths import (
     APP_NAME,
+    _portable_env_override,
     ensure_settings_file,
     get_legacy_settings_path,
     get_settings_dir_override,
@@ -172,13 +173,23 @@ class SettingsManager:
         self._check_environment_variables()
         self._collect_configuration_warnings()
 
-        if (
-            os.environ.get("LORA_MANAGER_PORTABLE", "0") == "1"
-            and not is_settings_dir_pinned()
-        ):
+        portable_override = _portable_env_override()
+        if portable_override is True and not is_settings_dir_pinned():
             if not self.settings.get("use_portable_settings"):
                 self.settings["use_portable_settings"] = True
                 self._save_settings()
+        elif portable_override is False and self.settings.get(
+            "use_portable_settings"
+        ):
+            # Explicit opt-out from a persisted portable mode: clear the flag so
+            # later runs go back to the shared settings directory instead of
+            # requiring a manual edit of settings.json.
+            logger.info(
+                "Clearing the persisted portable-mode flag because %s=0",
+                "LORA_MANAGER_PORTABLE",
+            )
+            self.settings["use_portable_settings"] = False
+            self._save_settings()
 
         if self._needs_initial_save:
             self._save_settings()