- bal/core/plugin_base.py: change chainname from frozen class attribute
to @classproperty so it reads constants.net.NET_NAME at runtime, fixing
regtest/testnet always downloading the mainnet executor list
- bal/core/willexecutors.py: remove module-level chainname capture;
all uses now read BalPlugin.chainname directly
- bal/gui/qt/dialogs.py: when BalBuildWillDialog detects no heirs,
return 'no_heirs' signal and show Heirs/Wizard/Close buttons (mirroring
the existing no-willexecutor pattern); add HeirsDialog with full
HeirListWidget (New Heir, Import, Export)