feat(bal): UI batch v0.3.9 + expired-will invalidate regression fix

UI batch (TASK A/B/C/D):
- (A) Clearer 'Will expired' message: shortened will id (8+8 chars) and
  readable UTC date instead of raw UNIX timestamp; shown in WARNING colour
  (orange) instead of ERROR (red) in the wizard, as it is an expected step.
- (A2) Split the orange message onto two lines via <br> (rendered as HTML by
  msg_warning) to avoid an overly long single line.
- (B) History tab labels (text only): inheritance txs -> 'BAL Inheritance
  transaction'; invalidate tx -> 'BAL Invalidate transaction'. Colours left
  to Electrum defaults.
- (C) New 'No will-executor TX' checkbox in plugin settings, bound to the
  existing NO_WILLEXECUTOR config (default ON, kept in sync with the wizard),
  with help text, and included in the settings reset action.
- (D) Wizard button label 'Create your will' -> 'Build Your Will'.

Regression fix (A3):
- When an heir was added to an already-expired will via the wizard, the
  automatic invalidation no longer triggered. Root cause: the inner
  check_will() after build_will() raised WillExpiredException and the handler
  returned (False, None), which never invalidated.
- Auto-opening the invalidation tx window from the closing wizard proved
  unreliable (the window ended up behind the main wallet on some window
  managers; a re-check loop could ask to invalidate repeatedly before the tx
  reached the mempool). The robust fix: close the wizard and show a clear
  instruction popup guiding the user to run 'Tools -> Invalidate' and then
  press 'Check' to finish the will. Reasoning documented in the code.

Version bumped to 0.3.9 (manifest.json, __init__.py, plugin_base.py, VERSION).
CHANGELOG entry #15 updated. Verified: py_compile OK, ruff clean (no new
errors), full test suite 239 passed.

bal/gui/qt/window.py

@@ -608,7 +608,8 @@ class BalWindow:
                 try:
                     self.check_will()
                     for wid, _w in self.willitems.items():
-                        self.wallet.set_label(wid, "BAL Transaction")
+                        # Label shown in Electrum's History tab for inheritance txs.
+                        self.wallet.set_label(wid, "BAL Inheritance transaction")
                     rebuilt_ok = True
                 except WillExpiredException as e:
                     self.invalidate_will()
@@ -699,7 +700,8 @@ class BalWindow:
                         "Please sign and broadcast this transaction to invalidate current will"
                     )
                 )
-                self.wallet.set_label(result.txid(), "BAL Invalidate")
+                # Label shown in Electrum's History tab for invalidate txs.
+                self.wallet.set_label(result.txid(), "BAL Invalidate transaction")
                 self.show_transaction(result)
             else:
                 self.show_message(_("No transactions to invalidate"))