Previously, when HTMX form submissions (profile update, password change)
returned a successful JSON response like {"message": "profile updated
successfully"}, the raw JSON was swapped into the target div as plain text.
The htmx:afterSwap listener in toast.ts only handled error responses.
Extend it to also intercept successful 2xx JSON responses that contain a
"message" field, showing a green success toast and clearing the raw JSON
from the target element. Only JSON responses are intercepted (checked via
Content-Type header), so legitimate HTML swaps are unaffected.