disable feed toolbar catchup button when catchup is not applicable

feedlist.js

@@ -77,6 +77,13 @@ function viewfeed(feed, skip, subop, doc) {
 	
 	disableContainerChildren("headlinesToolbar", false, doc);
 
+	var btnMarkAsRead = doc.getElementById("btnMarkFeedAsRead");
+
+	if (btnMarkAsRead && (feed < 0 || !isNumeric(feed))) {
+		btnMarkAsRead.disabled = true;
+		btnMarkAsRead.className = "disabledButton";
+	}
+
 //	notify("");
 
 }

tt-rss.js

@@ -276,6 +276,13 @@ function viewfeed(feed, skip, subop) {
 	
 	disableContainerChildren("headlinesToolbar", false, doc);
 
+	var btnMarkAsRead = document.getElementById("btnMarkFeedAsRead");
+
+	if (btnMarkAsRead && (feed < 0 || !isNumeric(feed))) {
+		btnMarkAsRead.disabled = true;
+		btnMarkAsRead.className = "disabledButton";
+	}
+
 //	notify("");
 
 }

tt-rss.php

@@ -110,7 +110,7 @@
 		&nbsp;Feed: <input class="button" type="submit"
 			onclick="javascript:viewCurrentFeed(0, 'ForceUpdate')" value="Update">
 
-		<input class="button" type="submit"
+		<input class="button" type="submit" id="btnMarkFeedAsRead"
 			onclick="javascript:viewCurrentFeed(0, 'MarkAllRead')" value="Mark as read">
 
 		</td>