Add Phase 4 master plan and install leaflet dependency

Write the flights-map master plan covering sub-plans 4A-4D.
Add leaflet + @types/leaflet to support the map canvas component.

package.json

@@ -35,6 +35,7 @@
     "i18next-icu": "^2.0.0",
     "i18next-resources-to-backend": "^1.0.0",
     "intl-messageformat": "^10.0.0",
+    "leaflet": "^1.9.4",
     "lru-cache": "^10.0.0",
     "react": "^18.2.0",
     "react-dom": "^18.2.0",
@@ -48,6 +49,7 @@
     "@testing-library/jest-dom": "^6",
     "@testing-library/react": "^16.3.2",
     "@testing-library/react-hooks": "^8.0.1",
+    "@types/leaflet": "^1.9.21",
     "@types/node": "^24.0.0",
     "@types/react": "^18.2.0",
     "@types/react-dom": "^18.2.0",