Enable vue/return-in-computed-property and add explicit fallback returns

Co-Authored-By: Codex <codex@openai.com>

eslint.config.mjs

@@ -118,7 +118,7 @@ export default [
     rules: {
       "vue/multi-word-component-names": "off",
       "vue/no-side-effects-in-computed-properties": "off",
-      "vue/return-in-computed-property": "off",
+      "vue/return-in-computed-property": "error",
       "vue/no-parsing-error": "error",
       "vue/valid-v-else": "error",
       "vue/no-deprecated-v-on-native-modifier": "error",