test: campaign characterization (7 tests)

- src/testHelpers.js: renderApp, createCampaignViaUI, selectCampaignByName
- App.characterization.test.js: createCampaign, addCharacter, updateCharacter, deleteCharacter, deleteCampaign + path namespace + bg url
- mock firestore writeBatch sync (was async, app no-await)

Locks path + payload shape per action. Refactor guard.

src/__mocks__/firebase/firestore.js

@@ -39,7 +39,7 @@ export async function addDoc(collRef, data) {
   MOCK_DB.set(path, clone(data));
   return { id, path };
 }
-export async function writeBatch(db) {
+export function writeBatch(db) {
   const ops = [];
   return {
     set: (r, d) => ops.push({ op: 'set', path: r.path, data: clone(d) }),