Correct the version and make the config merge deeply

The package version had been left at the scaffolded 0.1.0 while the released
tag was v5.4.0. Nothing in the code chose 0.1.0 -- it was npm-init's equivalent,
never updated -- and it made every version comparison meaningless: the window
title, the About dialog and the update check all read it.

ConfigManager merged a stored config over the defaults with dict.update(),
which is shallow. A config written by an older build carries partial "player"
and "feed" objects, and a shallow update replaces the whole nested default with
the partial one, so keys added since came back missing. The `or 15` and
`or "auto"` fallbacks at the call sites were load-bearing because of it. The
merge is now recursive, and keys present only in the stored file are kept so a
downgrade cannot destroy settings.

Configs now carry a config_version. A file written before 5.4.0 -- including
one inherited from an upstream YTSage install -- has its stored
check_app_updates cleared once, because that setting used to point at PyPI's
`ytsage` package and oop7/YTSage's releases, neither of which is this program.

The settings tab read a missing check_app_updates as enabled and persisted that
reading on OK, so merely opening Custom Options turned the checker back on.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ytsage/utils/ytsage_config_manager.py

@@ -72,6 +72,9 @@ class ConfigManager:
     _config_file: Path = APP_CONFIG_FILE
     _settings: Dict[str, Any] = {}
     _default_config: Dict[str, Any] = {
+        # Bumped whenever a stored config needs rewriting rather than merely
+        # merging; see _migrate(). Absent means "written before 5.4.0".
+        "config_version": 2,
         "download_path": str(USER_HOME_DIR / "Downloads"),
         "generic_mode": True,
         "speed_limit_value": None,
@@ -122,6 +125,56 @@ class ConfigManager:
         },
     }
 
+    @classmethod
+    def _deep_merge(cls, defaults: Dict[str, Any], stored: Dict[str, Any]) -> Dict[str, Any]:
+        """
+        Recursively layer `stored` on top of `defaults`.
+
+        A shallow `dict.update()` here would be a data-loss bug: a config file
+        written by an older build contains a *partial* "player" or "feed"
+        object, and updating shallowly replaces the whole nested default with
+        it -- so every key added since would come back missing.
+
+        Rules:
+        - dict + dict recurse
+        - anything else (lists, scalars, type mismatches) replaces wholesale
+        - keys present only in `stored` are kept, never pruned; a user who
+          downgrades must not have their settings destroyed by the older build
+        """
+        merged = copy.deepcopy(defaults)
+        for key, value in stored.items():
+            existing = merged.get(key)
+            if isinstance(value, dict) and isinstance(existing, dict):
+                merged[key] = cls._deep_merge(existing, value)
+            else:
+                merged[key] = copy.deepcopy(value)
+        return merged
+
+    @classmethod
+    def _migrate(cls, stored: Dict[str, Any]) -> bool:
+        """
+        Bring a stored config up to CONFIG_VERSION in place.
+
+        Returns True when something changed, so the caller knows to re-save.
+        Migrations must be idempotent and must never raise -- a config that
+        cannot be migrated is still a config the app has to start with.
+        """
+        changed = False
+        version = stored.get("config_version")
+
+        if not isinstance(version, int) or version < 2:
+            # Pre-5.4.0, and possibly inherited from an upstream YTSage
+            # install. Such a file carries "check_app_updates": true, which
+            # used to point at PyPI's `ytsage` package and oop7/YTSage's
+            # releases -- neither of which is this application. Dropping the
+            # stored value lets SageTube's own default decide.
+            if stored.pop("check_app_updates", None) is not None:
+                logger.info("Config migration: cleared inherited 'check_app_updates'.")
+            stored["config_version"] = 2
+            changed = True
+
+        return changed
+
     @classmethod
     def _load(cls) -> None:
         """
@@ -134,11 +187,21 @@ class ConfigManager:
                 try:
                     with open(cls._config_file, "r", encoding="utf-8") as f:
                         stored = json.load(f)
+                    if not isinstance(stored, dict):
+                        raise json.JSONDecodeError("config root is not an object", "", 0)
+
+                    migrated = False
+                    try:
+                        migrated = cls._migrate(stored)
+                    except Exception as e:
+                        logger.exception(f"Config migration failed, continuing unmigrated: {e}")
+
                     # Merge on top of defaults so keys added in newer versions
                     # exist without call sites needing `or <default>` fallbacks
-                    cls._settings = copy.deepcopy(cls._default_config)
-                    cls._settings.update(stored)
+                    cls._settings = cls._deep_merge(cls._default_config, stored)
                     logger.info("Config loaded from file.")
+                    if migrated:
+                        cls._save()
                 except json.JSONDecodeError:
                     cls._settings = copy.deepcopy(cls._default_config)
                     logger.warning("Config file corrupt, loaded defaults.")