feat: explain mobile evidence screenshots (Closes #827)

frontend/index.html

@@ -763,6 +763,10 @@
           <img id="create-issue-attachment-image" alt="Selected screenshot preview" />
           <span class="small" id="create-issue-attachment-meta"></span>
           <div class="issue-evidence-tray" id="create-issue-evidence-tray" role="toolbar" aria-label="Evidence screenshots" hidden></div>
+          <label class="issue-evidence-note" for="create-issue-evidence-note">
+            <span id="create-issue-evidence-note-label">Evidence note (optional)</span>
+            <textarea id="create-issue-evidence-note" maxlength="240" rows="2" disabled placeholder="What does this screenshot show?"></textarea>
+          </label>
           <div class="issue-evidence-review-actions" aria-label="Reorder selected screenshot">
             <button id="move-create-issue-attachment-earlier" type="button">Move earlier</button>
             <button id="remove-create-issue-attachment" type="button">Remove selected</button>