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/incidents/page.tsx

@@ -28,6 +28,17 @@ const FILTER_THRESHOLD: Record<SeverityFilter, number> = { all: -1, minor: 1, mo
 
 type SortMode = "recent" | "severity";
 
+// The Firestore client surfaces a missing composite index or an undeployed
+// ruleset as a raw multi-line string with a console URL in it — not something
+// to put in front of an operator. Collapse the known infra failures to a plain
+// line; pass anything else straight through so a real bug still shows.
+function friendlyIncidentsError(raw: string): string {
+  if (/requires an index|PERMISSION_DENIED|Missing or insufficient permissions|failed-precondition/i.test(raw)) {
+    return "Couldn't load incidents — the incidents database index isn't deployed on the server yet. This is a one-time backend deploy step (server-26 #13 / #51), not a problem with your data.";
+  }
+  return `Couldn't load incidents: ${raw}`;
+}
+
 function fmtTime(iso: string) {
   try { return new Date(iso).toLocaleTimeString([], { hour: "2-digit", minute: "2-digit" }); } catch { return iso; }
 }
@@ -286,7 +297,7 @@ export default function IncidentsPage() {
               recorded yet" over the top of it told the operator the radio was
               quiet when the page had simply failed to load — server-26#13. */}
           {filtered.length === 0 && error && (
-            <ErrorBanner message={`Couldn't load incidents: ${error}`} />
+            <ErrorBanner message={friendlyIncidentsError(error)} />
           )}
 
           {filtered.length === 0 && !error && (