fix(deps): maplibre-gl 6.15.0 past the attribution sanitizer advisory

GHSA-jrc7-96c5-q579 covers every maplibre-gl up to 6.4.0 and has no 5.x
release, so npm audit kept flagging the patched 5.24.0. Moving to 6.15.0
retires the local backport: patch-maplibre.mjs and its postinstall,
predev and prebuild hooks go, along with the Dockerfile copy and its
format-baseline entry. 6.x sanitizes attributions against an allow list.

What the major changes for TREK:
- ESM only, no default export: the engine module imports the namespace
  and hands that out as its default, so every caller stays as it was.
- The worker is no longer inlined, and inside a bundle MapLibre cannot
  find it next to its own module. The engine sets it from Vite's
  ?worker&url build before any map, the Leaflet bridge's included.
- WebGL2 only: the probe asks for webgl2 alone, so a WebGL1-only browser
  gets the raster basemap instead of a GPUInitializationError.
- 6.9 shapes right-to-left text itself and deprecated setRTLTextPlugin;
  the old plugin would replace that and could only run through eval in
  the module worker, so MapLibre no longer registers it. Mapbox does.

The lockfile moves maplibre-gl and its own dependencies only; the
Leaflet bridge already allows ^6.0.0 as its peer.

client/src/components/Map/MapViewGL.plannerWaypoints.test.tsx

@@ -1,5 +1,5 @@
 import { act } from '@testing-library/react';
-import maplibregl from 'maplibre-gl';
+import * as maplibregl from 'maplibre-gl';
 import React from 'react';
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
 import { render } from '../../../tests/helpers/render';
@@ -164,22 +164,21 @@ vi.mock('mapbox-gl', () => ({
 }));
 vi.mock('mapbox-gl/dist/mapbox-gl.css', () => ({}));
 
+// maplibre-gl 6 has no default export, so the fakes are named exports.
 vi.mock('maplibre-gl', () => ({
-  default: {
-    Map: vi.fn(function () {
-      return glMap;
-    }),
-    Marker: vi.fn(function (opts?: { element?: HTMLElement }) {
-      return glMarkers.make(opts);
-    }),
-    LngLatBounds: vi.fn(function () {
-      return glBounds.create();
-    }),
-    NavigationControl: vi.fn(),
-    Popup: vi.fn(function () {
-      return glPopup;
-    }),
-  },
+  Map: vi.fn(function () {
+    return glMap;
+  }),
+  Marker: vi.fn(function (opts?: { element?: HTMLElement }) {
+    return glMarkers.make(opts);
+  }),
+  LngLatBounds: vi.fn(function () {
+    return glBounds.create();
+  }),
+  NavigationControl: vi.fn(),
+  Popup: vi.fn(function () {
+    return glPopup;
+  }),
 }));
 vi.mock('maplibre-gl/dist/maplibre-gl.css', () => ({}));