Standardize usage of period in UI sentences

src/qml/Pages/AddChat/FindSomeone.qml

@@ -33,10 +33,10 @@ HBox {
                 let txt = qsTr("Unknown error - %1: %2").arg(type).arg(args)
 
                 if (type === "InvalidUserInContext")
-                    txt = qsTr("You cannot invite yourself!")
+                    txt = qsTr("You can't invite yourself!")
 
                 if (type === "UserNotFound")
-                    txt = qsTr("This user does not exist.")
+                    txt = qsTr("This user does not exist")
 
                 errorMessage.text = txt
             })

src/qml/Pages/Default.qml

@@ -1,5 +1,5 @@
 import "../Base"
 
 HNoticePage {
-    text: qsTr("Add or select a room to start.")
+    text: qsTr("No chat selected")
 }