refactor: extract state management into dedicated module and persist across restarts

Move state handling out of daemon and paths into a new state module.

- Introduce state.py for loading, writing, and maintaining state.json
- Persist recorded file metadata across daemon restarts
- Add cleanup of stale state entries when files are deleted
- Rename cleanup_old_files to cleanup_physical_files for clarity
- Ensure state file is created lazily if missing

rewind/paths.py

@@ -8,6 +8,7 @@ from importlib import resources
 
 APP_NAME = "rewind"
 USER_CONFIG = Path.home() / ".rewind.toml"
+STATE_NAME = "state.json"
 
 def load_config() -> dict:
     if USER_CONFIG.exists():
@@ -16,25 +17,4 @@ def load_config() -> dict:
 
     # fallback to packaged default
     with resources.files("rewind").joinpath("config.toml").open("rb") as f:
-        return tomllib.load(f)
-
-def state_dir() -> Path:
-    base = os.path.expanduser("~/.local/share")
-    path = Path(base) / APP_NAME
-    path.mkdir(parents=True, exist_ok=True)
-    return path
-
-def get_state_file_path() -> Path:
-    return state_dir() / "state.json"
-
-def load_state() -> dict:
-    if not get_state_file_path().exists():
-        return {"files": []}
-    with get_state_file_path().open() as f:
-        return json.load(f)
-
-def write_state(state: dict) -> None:
-    tmp = get_state_file_path().with_suffix(".tmp")
-    with tmp.open("w") as f:
-        json.dump(state, f, indent=2)
-    tmp.replace(get_state_file_path())  # atomic
+        return tomllib.load(f)