feat(will): invalidate signed will on postpone + add Server status column

Postpone safety (Strategy B):
- A signed/sent will carries an immutable locktime; postponing the delivery
  time previously did nothing, so a will-executor could still broadcast the
  old (earlier-locktime) transaction and execute the inheritance too early.
- core/will.py: add WillPostponedException and detect postpone by comparing
  the requested locktime against w.tx.locktime (the locktime frozen in the
  signed transaction) instead of the in-memory heir entry, which is updated
  together with the new value and would always compare equal.
- gui/qt/dialogs.py (BalBuildWillDialog.task_phase1, the real path used by
  Tools -> Prepare): handle WillPostponedException before NotCompleteWill;
  return (None, tx) to trigger sign + broadcast of the invalidation, then the
  user presses Prepare again to rebuild/re-sign/re-send (two explicit steps).
- gui/qt/window.py: mirror the branch in build_inheritance_transaction with an
  explanatory message; wording aligned to the 'Prepare' button.
- gui/qt/common.py: export WillPostponedException.
- A postpone on a will that was never signed/sent just rebuilds (no on-chain
  fee).

Server status column:
- gui/qt/lists.py: add a dedicated 'Server' column to PreviewList with an
  always-readable label and a tooltip (will-executor URL + state).
- gui/qt/theme.py: add server_status_text() and server_status_tooltip(),
  reusing the existing status flags.
- gui/qt/common.py: export the new theme helpers.

Docs: update README.md, bal/README.md and CHANGELOG_REFACTOR.md.

Tests: 182 passed; smoke + external-zip OK; ruff has no new real findings.

bal/gui/qt/lists.py

@@ -228,12 +228,14 @@ class PreviewList(MyTreeView, MessageBoxMixin):
         TXID = enum.auto()
         WILLEXECUTOR = enum.auto()
         STATUS = enum.auto()
+        SERVER = enum.auto()
 
     headers = {
         Columns.LOCKTIME: _("Locktime"),
         Columns.TXID: _("Txid"),
         Columns.WILLEXECUTOR: _("Will-Executor"),
         Columns.STATUS: _("Status"),
+        Columns.SERVER: _("Server"),
     }
 
     ROLE_HEIR_KEY = Qt.ItemDataRole.UserRole + 2000
@@ -385,6 +387,9 @@ class PreviewList(MyTreeView, MessageBoxMixin):
         if len(bal_tx.status) > 53:
             status = "...{}".format(status[-50:])
         labels[self.Columns.STATUS] = status
+        # Dedicated, always-readable label describing whether the inheritance
+        # transaction is actually stored on the will-executor servers.
+        labels[self.Columns.SERVER] = server_status_text(bal_tx)
 
         items = []
         for e in labels:
@@ -398,6 +403,13 @@ class PreviewList(MyTreeView, MessageBoxMixin):
 
             items[-1].setBackground(QColor(status_color(bal_tx)))
 
+        # Tooltip on the Server column: shows the will-executor URL (if any)
+        # plus the current server state, so the user can always inspect details.
+        try:
+            items[self.Columns.SERVER].setToolTip(server_status_tooltip(bal_tx))
+        except Exception as tip_err:
+            _logger.debug(f"server tooltip error: {tip_err}")
+
         row_count = self.model().rowCount()
         self.model().insertRow(row_count, items)
         if txid == current_key: