fix: don't pre-select already-imported files when entering source config

selectSource had an empty dependency array, causing a stale closure
where importedFilesBySource was always the initial empty Map. This
made the new-file filter select all files including imported ones.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/hooks/useImportWizard.ts

@@ -363,7 +363,7 @@ export function useImportWizard() {
 
       dispatch({ type: "SET_STEP", payload: "source-config" });
     },
-    [] // eslint-disable-line react-hooks/exhaustive-deps
+    [state.importedFilesBySource] // eslint-disable-line react-hooks/exhaustive-deps
   );
 
   const loadHeadersWithConfig = useCallback(