fix(clients): bump auto-generated email length to 10 chars

The "create" form opened with a 9-char random email default, the bulk
modal's random portion was only 6 chars, and the inbound-defaults seed
used 8 — all below the 10-char minimum we want for new clients. Bring
each generator to 10 so an unedited auto-generated email meets the
threshold without the user having to extend it.

frontend/src/pages/clients/ClientFormModal.tsx

@@ -191,7 +191,7 @@ export default function ClientFormModal({
     } else {
       setForm({
         ...emptyForm(),
-        email: RandomUtil.randomLowerAndNum(9),
+        email: RandomUtil.randomLowerAndNum(10),
         uuid: RandomUtil.randomUUID(),
         subId: RandomUtil.randomLowerAndNum(16),
         password: RandomUtil.randomLowerAndNum(16),