fix(clients): reject spaces, '/', '\' and control chars in client email

Client emails containing a slash broke the path-param routes
(edit/delete/view returned 404 / "client not found"), leaving stale
records that could only be cleared with manual SQLite edits. Validate
the email on both the backend (Create + Update, which also covers the
bulk paths) and the frontend (Zod) so these characters are rejected at
save time with a clear, localized message across all 13 locales.

Closes #4695

web/translation/en-US.json

@@ -646,6 +646,7 @@
       "subId": "Subscription ID",
       "online": "Online",
       "email": "Email",
+      "emailInvalidChars": "Email cannot contain spaces, '/', '\\', or control characters",
       "group": "Group",
       "groupDesc": "Logical label used to bucket related clients (e.g. team, customer, region). Filterable from the toolbar.",
       "groupPlaceholder": "e.g. customer-a",