fix(gui): read push messages delivered under data

src/gui/src/sw.js

@@ -82,7 +82,10 @@ function readPushPayload (pushData) {
     } catch ( _e ) {
         return null;
     }
-    let message = payload && typeof payload === 'object' ? payload.puter : null;
+    // Providers that only carry string data deliver it under `data`.
+    let message = payload && typeof payload === 'object'
+        ? (payload.puter ?? payload.data?.puter)
+        : null;
     if ( typeof message === 'string' ) {
         try {
             message = JSON.parse(message);

src/gui/src/sw.test.js

@@ -90,6 +90,13 @@ describe('service worker helpers', () => {
         expect(context.readPushPayload(pushData({ puter: JSON.stringify(message) }))).toEqual(message);
     });
 
+    it('reads the message from string data under `data.puter`', () => {
+        const message = { uid: 'n2', title: 'Hello' };
+        const payload = { data: { puter: JSON.stringify(message) }, from: '123', messageId: 'm1' };
+        expect(context.readPushPayload(pushData(payload))).toEqual(message);
+        expect(context.readPushPayload(pushData({ data: { other: 'x' } }))).toBeNull();
+    });
+
     it('reads nothing from a missing, garbled or misplaced payload', () => {
         expect(context.readPushPayload(null)).toBeNull();
         expect(context.readPushPayload({ json: () => { throw new SyntaxError('bad'); } })).toBeNull();