refactor: replace custom API fetch logic with requestJson utility across multiple pages

Updated the API fetching mechanism in various components to utilize the new requestJson function for improved consistency and error handling. This change affects the alerts, dashboard, data collection, filters, gre, network map, probes, recursive routes, route optimizer, servers, settings, traffic, and uptime pages.

shared/api/http-client.ts

@@ -0,0 +1,45 @@
+export class ApiClientError extends Error {
+  constructor(
+    message: string,
+    public readonly status: number,
+    public readonly payload?: unknown,
+  ) {
+    super(message)
+    this.name = "ApiClientError"
+  }
+}
+
+function trimBaseUrl(baseUrl: string): string {
+  return baseUrl.replace(/\/$/, "")
+}
+
+export async function requestJson<T>(
+  baseUrl: string,
+  path: string,
+  init?: RequestInit,
+): Promise<T> {
+  const hasBody = init?.body != null
+  const res = await fetch(trimBaseUrl(baseUrl) + path, {
+    ...init,
+    headers: {
+      ...(hasBody ? { "Content-Type": "application/json" } : {}),
+      ...(init?.headers ?? {}),
+    },
+  })
+
+  if (res.status === 204) return undefined as T
+
+  const payload = await res.json().catch(() => undefined)
+  if (!res.ok) {
+    const msg =
+      typeof payload === "object" &&
+      payload !== null &&
+      "error" in payload &&
+      typeof (payload as { error?: unknown }).error === "string"
+        ? (payload as { error: string }).error
+        : res.statusText
+    throw new ApiClientError(msg, res.status, payload)
+  }
+
+  return payload as T
+}