feat: send mobile deadline reminders (Closes #709)

frontend/index.html

@@ -129,6 +129,8 @@
             <label for="delivery-receipts"><input id="delivery-receipts" type="checkbox" /> Notify me when queued work finishes</label>
             <label class="push-update-control" for="push-updates"><input id="push-updates" type="checkbox" /> Notify me about new updates</label>
             <span class="small" id="push-update-status" role="status" aria-live="polite"></span>
+            <label class="push-update-control" for="push-deadlines"><input id="push-deadlines" type="checkbox" /> Notify me about deadlines</label>
+            <span class="small" id="push-deadline-status" role="status" aria-live="polite"></span>
             <button id="clear-offline-work" type="button">Clear offline work data</button>
             <button id="retry-offline-storage" type="button" hidden>Retry offline saving</button>
             <span class="small" id="offline-work-status" role="status" aria-live="polite"></span>