fix(push): tell iPhone Safari users to open the Home Screen app

iOS offers Web Push only to a web app opened from the Home Screen. In a
Safari tab the push card said the browser does not support push
notifications, and the step that fixes it was only in the docs. On an
iPhone or iPad outside the installed app the card now says to add
HealthLog to the Home Screen, open it from there and turn push on.

WebPushCard grows by two lines for the second sentence; its baseline
goes from 201 to 203, still below the 215 it had before the previous
change.

src/__tests__/baselines/source-function-lines.json

@@ -1,7 +1,7 @@
 {
   "$comment": "Production functions over 200 lines, keyed \"file :: name\", and their length. Maintained by src/__tests__/source-size-ratchet.test.ts.",
   "count": 344,
-  "total": 111493,
+  "total": 111495,
   "entries": {
     "app/achievements/page.tsx :: AchievementsPage": 206,
     "app/api/admin/backups/upload/route.ts :: apiHandler(arg)": 275,
@@ -224,7 +224,7 @@
     "components/settings/sharing-section.tsx :: ShareLinksCard": 260,
     "components/settings/sources-section.tsx :: SourcesSection": 601,
     "components/settings/thresholds-editor-section.tsx :: MetricRow": 241,
-    "components/settings/web-push-card.tsx :: WebPushCard": 201,
+    "components/settings/web-push-card.tsx :: WebPushCard": 203,
     "components/symptoms/symptom-entry-form.tsx :: SymptomEntryForm": 317,
     "components/symptoms/symptoms-section.tsx :: SymptomsSection": 205,
     "components/targets/range-bar.tsx :: RangeBar": 220,