fix: eslint findings

Remove dead code and unused imports, drop useless assignments, and tune
rules in eslint.config.js (underscore-prefixed unused names, intentional BOM/NBSP,
React Compiler rules from eslint-plugin-react-hooks 7 as warnings).

src/views/SeriesSettings.tsx

@@ -29,7 +29,7 @@ export default function SeriesSettings({
   embedded?: boolean;
 }) {
   const toast = useToast();
-  const [active, setActive] = useState<InvoiceSeries | null>(null);
+  const [, setActive] = useState<InvoiceSeries | null>(null);
   const [all, setAll] = useState<InvoiceSeries[]>([]);
   const [loading, setLoading] = useState(true);