Detect failed logins; fix map camera recenter, marker contrast, dark map tiles

- Client: server returns 200 + Status:"Error" on bad credentials; loginWith* now
  throw LoginError with the server message instead of faking a signed-in state
- Map: uncontrolled camera positioned once on load + explicit actions only, so
  Search/marker-tap/re-renders no longer snap back to the user's location
- Markers: always-dark high-contrast bubble + zone-colored dot (some zones report
  a white BackgroundColor -> was white-on-white)
- Dark mode now switches map tiles to CARTO dark-matter

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

app/app.json

@@ -44,6 +44,7 @@
     "extra": {
       "psEnvironment": "prodv2",
       "mapStyleUrl": "https://tiles.openfreemap.org/styles/liberty",
+      "mapStyleUrlDark": "https://basemaps.cartocdn.com/gl/dark-matter-gl-style/style.json",
       "unifiedPushDefaultDistributor": "io.heckel.ntfy",
       "debugHttp": true
     }