plugins: timelock_recovery: move checksum func to base class

diff --git a/electrum/plugins/timelock_recovery/qt.py b/electrum/plugins/timelock_recovery/qt.py
index 7b96a9a..2d17a5e 100644
--- a/electrum/plugins/timelock_recovery/qt.py
+++ b/electrum/plugins/timelock_recovery/qt.py
@@ -681,17 +681,6 @@ class Plugin(TimelockRecoveryPlugin):
 
         return bool(download_dialog.exec())
 
-    @classmethod
-    def _checksum(cls, json_data: dict[str, Any]) -> str:
-        # Assumes the values have a consistent json representation (not a key-value
-        # object whose fields can be ordered in multiple ways).
-        return hashlib.sha256(json.dumps(
-            sorted(json_data.items()),
-            skipkeys=False, ensure_ascii=False, check_circular=True,
-            allow_nan=True, cls=None, indent=None, separators=(',', ':'),
-            default=None, sort_keys=False,
-        ).encode()).hexdigest()[:8]
-
     def _save_recovery_plan_json(self, context: TimelockRecoveryContext, download_dialog: WindowModalDialog):
         try:
             # Open a Save As dialog to get the file path
@@ -728,7 +717,7 @@ class Plugin(TimelockRecoveryPlugin):
                     "recovery_outputs": [[tx_output.address, tx_output.value] for tx_output in context.recovery_tx.outputs()],
                 }
                 # Simple checksum to ensure the file is not corrupted by foolish users
-                json_data["checksum"] = self._checksum(json_data)
+                json_data["checksum"] = self.json_checksum(json_data)
                 json.dump(json_data, json_file, indent=2)
             download_dialog.show_message(_("File saved successfully"))
             context.recovery_plan_saved = True
@@ -766,7 +755,7 @@ class Plugin(TimelockRecoveryPlugin):
                     "cancellation_amount": context.cancellation_tx.output_value(),
                 }
                 # Simple checksum to ensure the file is not corrupted by foolish users
-                json_data["checksum"] = self._checksum(json_data)
+                json_data["checksum"] = self.json_checksum(json_data)
                 json.dump(json_data, f, indent=2)
             download_dialog.show_message(_("File saved successfully"))
             context.cancellation_plan_saved = True
diff --git a/electrum/plugins/timelock_recovery/timelock_recovery.py b/electrum/plugins/timelock_recovery/timelock_recovery.py
index 27cb679..9d42cf4 100644
--- a/electrum/plugins/timelock_recovery/timelock_recovery.py
+++ b/electrum/plugins/timelock_recovery/timelock_recovery.py
@@ -1,5 +1,8 @@
 from datetime import datetime
-from typing import TYPE_CHECKING, Callable, List, Optional, Sequence, Tuple
+import hashlib
+import json
+from typing import TYPE_CHECKING, Callable, List, Optional, Sequence, Tuple, Any
+
 from electrum.bitcoin import address_to_script
 from electrum.plugin import BasePlugin
 from electrum.transaction import PartialTxOutput, PartialTxInput, TxOutpoint
@@ -154,3 +157,14 @@ class TimelockRecoveryContext:
 class TimelockRecoveryPlugin(BasePlugin):
     def __init__(self, parent, config, name):
         BasePlugin.__init__(self, parent, config, name)
+
+    @classmethod
+    def json_checksum(cls, json_data: dict[str, Any]) -> str:
+        # Assumes the values have a consistent json representation (not a key-value
+        # object whose fields can be ordered in multiple ways).
+        return hashlib.sha256(json.dumps(
+            sorted(json_data.items()),
+            skipkeys=False, ensure_ascii=False, check_circular=True,
+            allow_nan=True, cls=None, indent=None, separators=(',', ':'),
+            default=None, sort_keys=False,
+        ).encode()).hexdigest()[:8]
diff --git a/tests/test_timelock_recovery.py b/tests/test_timelock_recovery.py
index 93b7b72..bde6381 100644
--- a/tests/test_timelock_recovery.py
+++ b/tests/test_timelock_recovery.py
@@ -1,15 +1,16 @@
 from io import StringIO
-import json
-import os, sys
+import os
+import sys
+
 from electrum.bitcoin import address_to_script
 from electrum.fee_policy import FixedFeePolicy
-from electrum.plugins.timelock_recovery.timelock_recovery import TimelockRecoveryContext
 from electrum.simple_config import SimpleConfig
 from electrum.storage import WalletStorage
 from electrum.transaction import PartialTxOutput
 from electrum.wallet import Wallet
 from electrum.wallet_db import WalletDB
-from electrum.plugins.timelock_recovery.qt import Plugin as TimelockRecoveryQtPlugin
+
+from electrum.plugins.timelock_recovery.timelock_recovery import TimelockRecoveryContext, TimelockRecoveryPlugin
 
 from . import ElectrumTestCase
 
@@ -125,7 +126,7 @@ class TestTimelockRecovery(ElectrumTestCase):
         # Non-ASCII characters must be serialized as-is (ensure_ascii=False),
         # not escaped as \uXXXX sequences, before hashing.
         json_data = {"wallet_name": "Ωmega Wörld Ñoño 日本語 中文 עברית العربية", "id": "abc-123"}
-        result = TimelockRecoveryQtPlugin._checksum(json_data)
+        result = TimelockRecoveryPlugin.json_checksum(json_data)
         self.assertEqual(result, "74674eca")
 
     def test_checksum_bip_example(self):
@@ -167,5 +168,5 @@ class TestTimelockRecovery(ElectrumTestCase):
           ],
           "metadata": "sig:825d6b3858c175c7fc16da3134030e095c4f9089c3c89722247eeedc08a7ef4f",
         }
-        result = TimelockRecoveryQtPlugin._checksum(json_data)
+        result = TimelockRecoveryPlugin.json_checksum(json_data)
         self.assertEqual(result, "92f8b3da")