This side effect was most likely a mistake, as:
- it is inconsistent with the other paths, e.g., when the pref_lang is available;
- a get_xxx() method having a side effect is generally not what one expects.

Nicoco authored Sep 15, 2025 and Philipp Hörist committed Sep 16, 2025

From https://xmpp.org/extensions/xep-0045.html#subject-mod

> Note: A message with a <subject/> and a <body/> or a <subject/> and a <thread/> is a legitimate message, but it SHALL NOT be interpreted as a subject change.

Fixes #168

- Use datetime instead of float for timestamps
- Be more strict with parsing
- Reduce handlers used and simplify code