refactor: simplify unsubscribe "not-allowed" message

Make error message more generic and less specific:
- Change title from "Action Not Allowed" to "Action not possible"
- Simplify message to cover broader error cases
- Fix typo: "performe" → "performed"
- Apply same changes to Croatian version

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

web-app/messages/en.json

@@ -484,8 +484,8 @@
       "unknown": "An error occurred while unsubscribing. Please try again or contact your landlord."
     },
     "not-allowed": {
-      "title": "Action Not Allowed",
+      "title": "Action not possible",
-      "message": "You can only unsubscribe from verified email addresses. This email address has not been verified yet or has already been unsubscribed."
+      "message": "The selected action cannot be performed or the passed information is invalid."
     }
   },
   "privacy-policy-page": {

web-app/messages/hr.json

@@ -481,8 +481,8 @@
       "unknown": "Došlo je do greške prilikom odjave. Molimo pokušajte ponovno ili kontaktirajte vašeg vlasnika nekretnine."
     },
     "not-allowed": {
-      "title": "Akcija Nije Dopuštena",
+      "title": "Akcija nije moguća",
-      "message": "Možete se odjaviti samo s potvrđenih email adresa. Ova email adresa još nije potvrđena ili je već odjavljena."
+      "message": "Odabrana akcija nije moguća ili zadani podaci nisu ispravni."
     }
   },
   "privacy-policy-page": {