docs(bal): rewrite CHECK ALIVE help text (DATA/RAW modes) + fix two typos (v0.3.8)

Rewrite the CHECK ALIVE help popup (ThresholdTimeWidget.help_text) to explain
both modes, keeping the <b>CHECK ALIVE</b> title bold:
- DATA mode: set the check-alive date; on wallet open, if the date has passed
  the plugin asks whether to postpone the inheritance.
- RAW mode: when less than this time is missing, ask to invalidate; failing to
  invalidate delivers the transactions to the heirs; kept the d/y suffix help.

Fix two typos within this help text: 'less then' -> 'less than',
'currrent' -> 'current'.

Bump version to 0.3.8 across the four version files. CHANGELOG entry #14.

bal/__init__.py

@@ -34,4 +34,4 @@ The plugin targets Electrum 4.7.2 (the last stable release exposing
 ``json_db.register_dict``) and PyQt6.
 """
 
-__version__ = "0.3.7"
+__version__ = "0.3.8"