ssb: Disallow rich text paste on Firefox. Didn't realize it doesn't support contenteditable='plaintext-only'.

apps/ssb.json

@@ -1,5 +1,5 @@
 {
 	"type": "tildefriends-app",
 	"emoji": "🐌",
-	"previous": "&tttJtdqA+iHWYVzZzA8jQdghIFjoVVO/2t6kHqCLnZI=.sha256"
+	"previous": "&CsZ2FBYY9awXNPPHAhTlrOHuKlGU6RXZJ3UTV3UQRIE=.sha256"
 }

apps/ssb/tf-compose.js

@@ -180,6 +180,9 @@ class TfComposeElement extends LitElement {
 				break;
 			}
 		}
+
+		event.preventDefault();
+		document.execCommand('insertText', false, event.clipboardData.getData('text/plain'));
 	}
 
 	async submit() {