Bag: hint and Clear selection stay visible under the list (t20)

The hint and Clear button were inside the scrolling bag list, so a long
Ctrl list scrolled them away. They now sit in a footer under the list, and
Clear is a full-width, highlighted "Clear selection" button.

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

src/shell.html

@@ -25,6 +25,7 @@
       </div>
       <div class="bar" id="bagbar"><i></i></div>
       <div id="baglist"></div>
+      <div id="bagfoot"></div>
     </div>
   </div>
   <div class="corner bl">