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/README.md

@@ -1,2 +1,23 @@
 # BalPlugin
 Bitcoin After Life Electrum Plugin
+
+Free and decentralized Bitcoin inheritance support for Electrum: build
+time-locked "will" transactions that transfer your funds to your heirs if you
+stop refreshing them (dead-man's switch), optionally relayed by will-executor
+servers.
+
+## Key behaviours
+
+- **Anticipate / postpone safety**: changing the delivery time of an
+  already-signed will is handled safely. Postponing a signed/sent will first
+  asks you to invalidate the old transaction on-chain (so a will-executor can
+  never broadcast the earlier-locktime transaction and execute the inheritance
+  too early), then lets you rebuild and re-send the new one via
+  **Tools → Prepare**.
+- **"Server" column**: the will transaction list shows whether each transaction
+  is actually stored on the will-executor servers
+  (`Confirmed on server`, `Sent (not checked)`, `Send failed`,
+  `Not on server`, `Signed (not sent)`, `Not sent`), with a tooltip showing the
+  will-executor URL.
+
+See the top-level [`README.md`](../README.md) for installation and testing.