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/common.py

@@ -69,11 +69,11 @@ from ...core.will import (AmountException, HeirChangeException,
                           NotCompleteWillException, NoWillExecutorNotPresent,
                           TxFeesChangedException, Will,
                           WillexecutorChangeException, WillExecutorNotPresent,
-                          WillExpiredException, WillItem)
+                          WillExpiredException, WillItem, WillPostponedException)
 from ...core.willexecutors import Willexecutors
 
 # --- Presentation helpers ---
-from .theme import status_color
+from .theme import server_status_text, server_status_tooltip, status_color
 from .window_utils import (bring_to_front, show_modal, show_on_top,
                            stop_thread, top_level_of)