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/components/MapView.tsx

@@ -24,6 +24,17 @@ L.Icon.Default.mergeOptions({
   shadowUrl: "https://unpkg.com/leaflet@1.9.4/dist/images/marker-shadow.png",
 });
 
+// ── Basemap tiles ─────────────────────────────────────────────────────────────
+// Default is CARTO's keyless dark raster basemap — no token, fits the dark UI.
+// Overridable via NEXT_PUBLIC_MAP_TILE_URL so a keyed style (a CARTO account
+// style, MapTiler, Mapbox, …) can be dropped in for prod without a code change.
+// Whatever is supplied must use Leaflet's {s}/{z}/{x}/{y}{r} placeholder scheme.
+const MAP_TILE_URL =
+  process.env.NEXT_PUBLIC_MAP_TILE_URL ||
+  "https://{s}.basemaps.cartocdn.com/dark_all/{z}/{x}/{y}{r}.png";
+const MAP_TILE_ATTRIBUTION =
+  '&copy; <a href="https://www.openstreetmap.org/copyright">OpenStreetMap</a> contributors &copy; <a href="https://carto.com/">CARTO</a>';
+
 // ── Colour ────────────────────────────────────────────────────────────────────
 // Severity is the only hue on this map — see UI_REDESIGN.md §2.3. Incident
 // type is carried by the glyph knocked out of the pin, never by colour, and
@@ -540,14 +551,14 @@ export default function MapView({ nodes, activeCalls, incidents = [], calls = []
           {/* Base layers */}
           <LayersControl.BaseLayer checked name="Dark">
             <TileLayer
-              url="https://{s}.basemaps.cartocdn.com/dark_all/{z}/{x}/{y}{r}.png"
-              attribution='&copy; <a href="https://carto.com/">CARTO</a>'
+              url={MAP_TILE_URL}
+              attribution={MAP_TILE_ATTRIBUTION}
             />
           </LayersControl.BaseLayer>
           <LayersControl.BaseLayer name="Light">
             <TileLayer
               url="https://{s}.basemaps.cartocdn.com/light_all/{z}/{x}/{y}{r}.png"
-              attribution='&copy; <a href="https://carto.com/">CARTO</a>'
+              attribution={MAP_TILE_ATTRIBUTION}
             />
           </LayersControl.BaseLayer>
           <LayersControl.BaseLayer name="Streets">