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

[kiwipaulrob]

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.