fix(push): remove this browser's subscription on sign-out

Signing out ended the session and wiped the caches but left the Web
Push subscription in place, in the browser and on the server. The
browser kept showing the signed-out account's medication reminders,
also after someone else signed in on it, and the next account's
Settings showed push as already on.

Sign-out now runs the same steps as "Disable push" before the logout
call: DELETE the server row while the session is still valid, then
unsubscribe in the browser. Both callers share one helper. A failure
there does not block signing out. With the dispatcher no longer
counting "no subscription" as a failed send, signing out does not
start the five-failure countdown on the account's channel.

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": 111507,
+  "total": 111493,
   "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": 215,
+    "components/settings/web-push-card.tsx :: WebPushCard": 201,
     "components/symptoms/symptom-entry-form.tsx :: SymptomEntryForm": 317,
     "components/symptoms/symptoms-section.tsx :: SymptomsSection": 205,
     "components/targets/range-bar.tsx :: RangeBar": 220,