Added TypeScript for frontend

Added type definitions for components, functions, data
Added guards for network responses
fixes #8

front/src/components/LocationMarker.tsx

@@ -0,0 +1,35 @@
+import { Marker, Popup, useMapEvents } from "react-leaflet"
+import { LatLng } from 'leaflet'
+import { SetState } from "../utils/types"
+
+type LocationMarkerProps = {
+    address: string,
+    position: LatLng,
+    setPosition: SetState<LatLng>
+}
+
+const LocationMarker = ({ address, position, setPosition }: LocationMarkerProps) => {
+
+    const map = useMapEvents({
+        dragend: () => {
+            setPosition(map.getCenter())
+        },
+        zoomend: () => {
+            setPosition(map.getCenter())
+        },
+        resize: () => {
+            setPosition(map.getCenter())
+        }
+    })
+
+    return (
+        <Marker position={position}>
+            <Popup>
+                {address}
+                {position.lat.toFixed(4)}, {position.lng.toFixed(4)}
+            </Popup>
+        </Marker>
+    )
+}
+
+export default LocationMarker