4.0.0b3

Bug fixes

• Setup no longer crashes on mixed delivery timestamps — some accounts hit ConfigEntryNotReady on every refresh when one delivered parcel carried a Z-suffixed timestamp and another did not. The integration now normalises both shapes.

What's new

• Letter image entities keep a meaningful "last updated" — the entity's state is the parsed letter date instead of HA's boot time, so restarting Home Assistant no longer makes every announced letter look like it just arrived.
• Letter image entities expose more attributes — each image.postnl_..._letter_<title> entity now carries id, title, date and unread, mirroring the per-letter shape the sensor.postnl_letters letters attribute and the postnl_letter_announced event already use.

No breaking changes — purely additive on top of 4.0.0b2.