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/core/willexecutors.py

@@ -23,7 +23,7 @@ from electrum.i18n import _
 from electrum.logging import get_logger
 from electrum.network import Network
 
-from .plugin_base import BalPlugin
+from .plugin_base import BalPlugin, get_version
 
 # Per-request timeout (seconds) for interactive operations (ping / info /
 # list download).  These fail fast (no retries) so a dead server does not
@@ -272,7 +272,7 @@ class Willexecutors:
             raise Exception("You are offline.")
         _logger.debug(f"<-- {method} {url} {data}")
         headers = {}
-        headers["user-agent"] = f"BalPlugin v:{BalPlugin.__version__}"
+        headers["user-agent"] = f"BalPlugin v:{get_version()}"
         headers["Content-Type"] = "text/plain"
         if not handle_response:
             handle_response = Willexecutors.handle_response