Add frontend POI management and travel time display

POIManager component in FilterPanel for creating/deleting POIs and
triggering distance calculations. PropertyCard shows travel time badges
(walk/cycle/transit) per POI. Map renders POI locations as red markers.
API client extended with POST body support for POI endpoints.

frontend/src/services/apiClient.ts

@@ -6,6 +6,7 @@ import { ApiError } from '@/types';
 export interface RequestOptions {
   method?: 'GET' | 'POST' | 'PUT' | 'DELETE';
   params?: Record<string, string | number | boolean | Date | undefined>;
+  body?: unknown;
 }
 
 /**
@@ -35,7 +36,7 @@ export async function apiRequest<T>(
   endpoint: string,
   options: RequestOptions = {}
 ): Promise<T> {
-  const { method = 'GET', params } = options;
+  const { method = 'GET', params, body } = options;
 
   let url = endpoint;
   if (params) {
@@ -45,13 +46,19 @@ export async function apiRequest<T>(
     }
   }
 
-  const response = await fetch(url, {
+  const fetchOptions: RequestInit = {
     method,
     headers: {
       Authorization: `Bearer ${user.accessToken}`,
       'Content-Type': 'application/json',
     },
-  });
+  };
+
+  if (body !== undefined) {
+    fetchOptions.body = JSON.stringify(body);
+  }
+
+  const response = await fetch(url, fetchOptions);
 
   if (!response.ok) {
     throw new ApiError(`Error: ${response.status}`, response.status);