frontend: fix map stacking + honest infra error states

From a live review of drb.cusano.net.

MapView.tsx / globals.css:
- The Leaflet map painted above the sticky Nav (z-40) and modal overlays, so
  on Live the account dropdown opened *behind* the map. Pin .leaflet-container
  to its own stacking context (position:relative; z-index:0) — keeps Leaflet's
  internal pane order, drops the whole map below app chrome. The map's own
  overlay UI (legend, rail, clock, fit-all) is outside .leaflet-container and
  unaffected. Chosen over raising Nav's z-index, which would float the sticky
  header over modal backdrops on ~7 pages.
- Basemap: the "Dark" tile URL is already CARTO's keyless dark raster (so a
  prod "API KEY REQUIRED" watermark is a stale build or CARTO rate-limiting
  the origin, not this code). Add NEXT_PUBLIC_MAP_TILE_URL as a build-time
  override so a keyed style drops in without a code change; add the OSM
  attribution the keyless CARTO tiles require.

incidents/page.tsx, alerts/page.tsx:
- Both dumped raw Firestore "requires an index / PERMISSION_DENIED" strings
  (with a console.firebase URL) straight into the UI when the composite
  indexes aren't deployed (server-26 #13/#51). Collapse those known infra
  failures to a plain sentence; any other error passes through verbatim so a
  real bug still shows. alerts also now surfaces the events-query error at
  all — it was swallowed, showing a false "No alerts triggered yet." on a
  public-safety screen.

onboarding/page.tsx: stale comment (/dashboard -> "/").

Untypechecked (no node/npm locally); presentational only — one string
helper, one added error branch, a CSS rule, two tile-URL constants, a
comment. next build in deploy.yml gates it.

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

drb-frontend/app/onboarding/page.tsx

@@ -43,7 +43,7 @@ export default function OnboardingPage() {
       // Firebase custom claims only show up in a *freshly fetched* ID token —
       // getIdTokenResult(true) inside refreshClaims forces that fetch, then
       // AuthProvider's own state (orgId) updates and the effect above
-      // redirects to /dashboard.
+      // redirects to "/" (Live).
       await refreshClaims();
     } catch (err) {
       setError(err instanceof Error ? err.message : "Could not set up your organization. Try again.");