Review follow-ups (Greptile):
- A source switch while `get_account_info` is in flight no longer hands
playlist A's status or clock to playlist B: the store is patched only
while the asking playlist is still selected, the timezone is persisted
under the asking playlist's id regardless, and a late failure cannot mark
the newly selected playlist unavailable.
- `parseNaiveUtcMs` reads the constructed date back, so out-of-range panel
strings (`2026-13-01 25:00:00`) are rejected instead of silently rolling
over into a real instant.
- Document that a clock-derived fixed offset is a DST-less snapshot, refreshed
by every account-info check and only ever used for non-standard servers.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>