Add an add-reaction button to the reaction row (#78)

Adding to a message that already had reactions meant going back up to
the hover row's 🙂. A "+🙂" button now sits at the end of the row of
reaction pills, always visible once the message has at least one
reaction, and opens the same emoji picker.

The picker anchors to the left edge of the pill row rather than the
button: the button moves right with every added reaction, and a picker
hanging off it ran past the edge of a phone screen. Both buttons share
the picker's open state, so only one picker is ever open at a time.

MessageList had no tests; this adds a few covering the new button.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

USER_GUIDE.md

@@ -133,7 +133,9 @@ you'll see an error before it uploads.
 Hover over a message and click the 🙂 icon in its action row to react
 with an emoji. Reactions from everyone appear as small pills under the
 message with a count; click an existing pill to add or remove your own
-reaction to it. Hovering a pill shows who reacted.
+reaction to it. Hovering a pill shows who reacted. Once a message has
+reactions, a small **+🙂** button at the end of the row of pills lets you
+add a different emoji without going back to the action row.
 
 Anyone can add a custom emoji: open the emoji picker (the 🙂 button, either
 in the composer or on a message) and click **+ Add** in the **Custom**