chore: enhance frontend configuration for backend integration

- Added environment variables for backend URLs in the Dockerfile to facilitate dynamic configuration.
- Updated Next.js configuration to include rewrites for health and API endpoints, improving backend connectivity.
- Modified settings page to handle backend URL locking and normalization, enhancing user experience and data integrity.

lib/backend-url.ts

@@ -0,0 +1,32 @@
+export const LOCAL_DEFAULT_BACKEND_URL = "http://localhost:8000"
+
+export type ConfiguredBackendUrl =
+  | { kind: "local" }
+  | { kind: "fixed"; url: string }
+  | { kind: "same-origin" }
+
+export function configuredBackendUrl(): ConfiguredBackendUrl {
+  const raw = process.env.NEXT_PUBLIC_BACKEND_URL
+  if (raw === undefined || raw === "local") return { kind: "local" }
+  if (raw === "" || raw === "same-origin") return { kind: "same-origin" }
+  return { kind: "fixed", url: raw.replace(/\/$/, "") }
+}
+
+export function defaultDataSourceMode(): "mock" | "live" {
+  return process.env.NEXT_PUBLIC_DEFAULT_DATA_SOURCE === "live" ? "live" : "mock"
+}
+
+export function isBackendUrlLocked(): boolean {
+  const cfg = configuredBackendUrl()
+  return cfg.kind === "same-origin" || cfg.kind === "fixed"
+}
+
+export function resolveStoredBackendUrl(stored: string | null): string {
+  const cfg = configuredBackendUrl()
+  if (cfg.kind === "fixed") return cfg.url
+  if (cfg.kind === "same-origin" && typeof window !== "undefined") {
+    return window.location.origin
+  }
+  const trimmed = stored?.trim().replace(/\/$/, "")
+  return trimmed || LOCAL_DEFAULT_BACKEND_URL
+}