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/window.py

@@ -529,6 +529,29 @@ class BalWindow:
                 return
             except NoHeirsException:
                 return
+            except WillPostponedException as e:
+                # The will was already signed/sent and is being postponed.
+                # We do NOT rebuild automatically: the user must first sign and
+                # broadcast the invalidation tx (so the old, earlier-locktime tx
+                # can never be used by a will-executor), then press "Prepare"
+                # again
+                # to create the new postponed inheritance.
+                _logger.info(f"will postponed: {e}")
+                self.show_message(
+                    _(
+                        "This inheritance was already signed/sent to "
+                        "will-executors and you are postponing it.\n\n"
+                        "The previously committed coins must be invalidated "
+                        "on-chain FIRST, otherwise a will-executor could "
+                        "broadcast the old (earlier) transaction and execute "
+                        "the inheritance too early.\n\n"
+                        "Please sign and broadcast the invalidation transaction "
+                        "now, then press 'Prepare' again to create the new "
+                        "(postponed) inheritance."
+                    )
+                )
+                self.invalidate_will()
+                return
             except NotCompleteWillException as e:
                 _logger.info("{}:{}".format(type(e), e))
                 message = False