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>

src/ccma/ui/gnucash_import_dialog.py

@@ -210,7 +210,7 @@ class GnuCashImportDialog(tk.Toplevel):
         }
         self.account_combo.configure(values=list(self.account_by_label))
         if self.account_by_label and self.account_var.get() not in self.account_by_label:
-            self.account_var.set(next(iter(self.account_by_label)))
+            self.account_var.set(self._preferred_account_label() or next(iter(self.account_by_label)))
         self.selected_guids.clear()
         if self.account_var.get():
             self._account_selected()
@@ -221,6 +221,27 @@ class GnuCashImportDialog(tk.Toplevel):
             self.config.gnucash_path = path_text
             self.config.save()
 
+    def _current_file_key(self) -> str | None:
+        path_text = self.file_var.get().strip()
+        if not path_text:
+            return None
+        path = Path(path_text).expanduser()
+        if not path.is_file():
+            return None
+        return str(path.resolve())
+
+    def _preferred_account_label(self) -> str | None:
+        file_key = self._current_file_key()
+        if file_key is None:
+            return None
+        last_guid = self.config.gnucash_last_accounts.get(file_key)
+        if not last_guid:
+            return None
+        return next(
+            (label for label, account in self.account_by_label.items() if account.guid == last_guid),
+            None,
+        )
+
     def _account_selected(self) -> None:
         account = self.account_by_label.get(self.account_var.get())
         if account is None:
@@ -236,6 +257,10 @@ class GnuCashImportDialog(tk.Toplevel):
         self.transactions = [item for item in all_transactions if item.amount > 0]
         self.selected_guids.clear()
         self._render_transactions()
+        file_key = self._current_file_key()
+        if file_key is not None and self.config.gnucash_last_accounts.get(file_key) != account.guid:
+            self.config.gnucash_last_accounts[file_key] = account.guid
+            self.config.save()
 
     def _filtered_transactions(self) -> list[GnuCashTransaction]:
         needle = self.description_filter_var.get().strip().casefold()