fix(settings): write settings file atomically via temp + rename (#285)

## Problem

`BaseSettings._save` (`sendspin/settings.py`, shared by client and serve
settings) rewrites the live JSON file in place with `write_text`. A kill
mid-write truncates it, and on next start the `JSONDecodeError` handler
logs a warning and silently falls back to defaults -- volume, name,
interface and hooks all revert with no obvious cause.

## Fix

Write-temp-then-rename: content goes to a `.tmp` sibling, is fsynced,
then `os.replace()` swaps it into place atomically. A crash can now only
leave the previous good file behind. Same `OSError` handling as before.

## Verification

- Full suite (`pytest tests/`, excluding serve + audio_devices): 123
passed
- `ruff check` clean

Covers both `ClientSettings` and `ServeSettings` since they share
`_save`.

Author: kiwipaulrob

sendspin/settings.py

@@ -105,7 +105,13 @@ def _save(self) -> None:
             return
         try:
             self._settings_file.parent.mkdir(parents=True, exist_ok=True)
-            self._settings_file.write_text(json.dumps(self.to_dict(), indent=2) + "\n")
+            # Write atomically: a kill mid-write must not truncate the live
+            # file (a truncated file falls back to defaults on next load).
+            tmp_file = self._settings_file.with_suffix(self._settings_file.suffix + ".tmp")
+            tmp_file.write_text(json.dumps(self.to_dict(), indent=2) + "\n")
+            with open(tmp_file, "rb") as f:
+                os.fsync(f.fileno())
+            os.replace(tmp_file, self._settings_file)
             logger.debug("Saved settings to %s", self._settings_file)
         except OSError as e:
             logger.warning("Failed to save settings to %s: %s", self._settings_file, e)