v0.4.0: Anonymous Mode + zone-location mirror; free check-in on any zone

Anonymous Mode — "Park without signing in" on the login screen (with a popup of
what works vs needs a login). Anonymous users browse parking areas from our
mirror, see labels, and start free check-in timers; paying, sessions, and
account screens prompt to sign in. AuthContext gains an 'anonymous' status +
enterAnonymous/requireLogin.

Zone mirror — server gains a `zones` table + public GET /api/zones and admin
POST /api/zones/sync. Signed-in admins push the zones they pull (authed) from
ParkSmarter after each map search, so anonymous users can read areas without a
ParkSmarter login. Map/Scan read the mirror when anonymous.

Also: the free "Check in" button is now always available with a 2h/3h/4h picker
(no longer gated on a prior label) — fixes "couldn't start a timer on a free
zone". CORS probe confirmed ParkSmarter allows any origin but only Content-Type,
so a future PWA can't auth to it — the mirror is what makes anonymous browsing
(and a PWA) possible.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/app.json

@@ -3,14 +3,14 @@
     "name": "BigBrainParking",
     "slug": "bigbrainparking",
     "scheme": "bigbrainparking",
-    "version": "0.3.0",
+    "version": "0.4.0",
     "orientation": "portrait",
     "userInterfaceStyle": "automatic",
     "newArchEnabled": true,
     "icon": "./assets/icon.png",
     "android": {
       "package": "top.mowden.bigbrainparking",
-      "versionCode": 17,
+      "versionCode": 18,
       "edgeToEdgeEnabled": true,
       "adaptiveIcon": {
         "foregroundImage": "./assets/adaptive-icon.png",