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/dialogs.py

@@ -595,6 +595,20 @@ class BalBuildWillDialog(BalDialog):
             return None, Will.invalidate_will(
                 self.bal_window.willitems, self.bal_window.wallet, fee_per_byte
             )
+        except WillPostponedException as e:
+            # An already signed/sent will is being postponed.  Like an expired
+            # will, the previously committed coins must be invalidated on-chain
+            # FIRST (otherwise a will-executor could broadcast the old,
+            # earlier-locktime tx and execute the inheritance too early).  We
+            # return (None, tx) so phase 2 asks the user to sign and broadcast
+            # the invalidation; afterwards the user presses Prepare again to
+            # rebuild the new (postponed) inheritance.
+            _logger.debug(f"postponed {e}")
+            self.msg_set_checking(_("Postponed: invalidating old will"))
+            fee_per_byte = self.bal_window.will_settings.get("baltx_fees", 1)
+            return None, Will.invalidate_will(
+                self.bal_window.willitems, self.bal_window.wallet, fee_per_byte
+            )
         except NoHeirsException as e:
             _logger.debug("no heirs")
             self.msg_set_checking("No Heirs")