Added staleness indicator to the output

src/web/html/index.html

@@ -172,6 +172,7 @@
                             </div>
                             <div class="io-info" id="output-info"></div>
                             <div class="io-info" id="output-selection-info"></div>
+                            <span id="stale-indicator" title="The output is stale.&#10;The input or recipe has changed since this output was generated. Bake again to get the new value.">&#x1F551;</span>
                         </div>
                         <div class="textarea-wrapper">
                             <div id="output-highlighter" class="no-select"></div>