Remember the last-used GnuCash account per file

AppConfig now keeps a gnucash_last_accounts map (file path -> account
guid). When the import dialog opens a file it already knows, it
auto-selects whichever account was picked last time for that specific
file instead of always defaulting to the first one alphabetically;
picking a different account updates and persists the mapping right away.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/test_config.py

@@ -48,3 +48,27 @@ def test_store_path_can_be_overridden_from_ccma_environment(tmp_path, monkeypatc
     monkeypatch.setenv("CCMA_STORE", str(tmp_path / "store"))
 
     assert load_config().store_path == str(tmp_path / "store")
+
+
+def test_gnucash_last_account_per_file_round_trips(tmp_path, monkeypatch) -> None:
+    monkeypatch.setenv("CCMA_CONFIG_DIR", str(tmp_path / "config"))
+    first_file = str(tmp_path / "verein.gnucash")
+    second_file = str(tmp_path / "spenden.gnucash")
+    config = AppConfig(
+        gnucash_last_accounts={first_file: "bank-guid", second_file: "cash-guid"}
+    )
+    config.save()
+
+    loaded = load_config()
+    assert loaded.gnucash_last_accounts == {first_file: "bank-guid", second_file: "cash-guid"}
+
+
+def test_gnucash_last_accounts_defaults_to_empty_dict_for_malformed_value(tmp_path, monkeypatch) -> None:
+    monkeypatch.setenv("CCMA_CONFIG_DIR", str(tmp_path / "config"))
+    config_path = tmp_path / "config" / "config.json"
+    config_path.parent.mkdir(parents=True, exist_ok=True)
+    config_path.write_text(
+        json.dumps({"schema_version": 1, "gnucash_last_accounts": "not-a-dict"}), encoding="utf-8"
+    )
+
+    assert load_config().gnucash_last_accounts == {}