It shows "Send a message..." instead of "Send an encrypted message..."

**Describe the solution you'd like**
I think the message shield's placeholder text should be "Send an encrypted message..." like in the Windows client to be consistent and reassuring.

Search the iOS client source for the exact placeholder text "Send a message..."; the issue names no file or test. Compare the wording with the Windows client reference, update the placeholder to "Send an encrypted message...", and verify the message shield displays the new text.

Written by the indexing model from the issue text.