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/theme.py

@@ -57,3 +57,41 @@ def status_color(will_item) -> str:
         return "#2bc8ed"  # blue - signed
     else:
         return _DEFAULT_COLOR
+
+
+def server_status_text(will_item) -> str:
+    """Return a short, human-readable label describing the state of a will
+    item on the will-executor servers (the online inheritance backup).
+
+    This is shown in the dedicated "Server" column of the transaction list so
+    the user always knows whether each inheritance transaction is actually
+    stored on the will-executor servers, regardless of the row colour.
+    """
+    from electrum.i18n import _
+
+    if will_item.get_status("CHECK_FAIL") and not will_item.get_status("CHECKED"):
+        return _("Not on server")
+    if will_item.get_status("CHECKED"):
+        return _("Confirmed on server")
+    if will_item.get_status("PUSH_FAIL"):
+        return _("Send failed")
+    if will_item.get_status("PUSHED"):
+        return _("Sent (not checked)")
+    if will_item.get_status("COMPLETE"):
+        return _("Signed (not sent)")
+    return _("Not sent")
+
+
+def server_status_tooltip(will_item) -> str:
+    """Return a detailed tooltip for the "Server" column, including the
+    will-executor URL (if any) and the current server state."""
+    from electrum.i18n import _
+
+    url = None
+    we = getattr(will_item, "we", None)
+    if we:
+        url = we.get("url")
+    state = server_status_text(will_item)
+    if url:
+        return "{}: {}\n{}".format(_("Will-Executor"), url, state)
+    return "{}\n{}".format(_("No will-executor"), state)