v0.6.1: read version from manifest.json (single source of truth)

Read the plugin version at runtime from bal/manifest.json via
importlib.resources (zip-safe, works from inside a zip on Windows).
Removes the four-file duplication (VERSION, __init__.py, plugin_base.py,
manifest.json) and deletes the bal/VERSION file.

- bal/core/plugin_base.py: new get_version() function, BalPlugin.version
  is now a @property, removed hardcoded __version__
- bal/__init__.py: removed __version__, replaced with a comment
- bal/core/willexecutors.py: user-agent uses get_version()
- bal/gui/qt/dialogs.py, widgets.py: call sites use .version property
- tests/test_version_source.py: 7 tests incl. zip-safe subprocess check
- HANDOFF.md: updated to single-source-of-truth documentation
- CHANGELOG.md: entries 43-46
- bal/manifest.json: version bumped to 0.6.1

PR #4 (branch bitcoinafterlife-patch-5).

bal/__init__.py

@@ -36,4 +36,7 @@ The plugin supports Electrum 4.7.2 and 4.8.0 with PyQt6.  Electrum 4.8.0 removed
 available and adapts, so both releases keep working.
 """
 
-__version__ = "0.5.18"
+# The plugin version is NOT defined here. It lives only in ``bal/manifest.json``
+# (the single source of truth) and is read at runtime via ``get_version()`` in
+# ``bal/core/plugin_base.py`` (exposed as the ``BalPlugin.version`` property).
+# Keeping a hardcoded ``__version__`` here would just be a stale duplicate.