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/gui/qt/dialogs.py

@@ -1533,7 +1533,7 @@ class BalBuildWillDialog(BalDialog):
                 "BEGIN:VCALENDAR",
                 "VERSION:2.0",
                 "PRODID:-//Bitcoin After Life//Electrum Plugin/"
-                f"{BalPlugin.__version__}",
+                f"{self.bal_window.bal_plugin.version}",
             ]
 
             total = len(offsets)