build(docker): add runtime-configured frontend image

frontend/src/api/runtime-config.test.ts

@@ -0,0 +1,17 @@
+import { describe, expect, it } from "vitest";
+
+import { backendBaseUrl, resolveBackendUrl } from "./runtime-config";
+
+describe("resolveBackendUrl", () => {
+  it("uses the runtime-injected backend URL", () => {
+    expect(resolveBackendUrl({ backendBaseUrl: "/api" })).toBe("/api");
+  });
+
+  it("falls back to the Vite backend URL", () => {
+    expect(resolveBackendUrl(undefined)).toBe(import.meta.env.VITE_BACKEND_URL ?? "");
+  });
+
+  it("preserves the client default when Vite has no configured backend", () => {
+    expect(backendBaseUrl).toBe(import.meta.env.VITE_BACKEND_URL ?? "http://localhost:8787");
+  });
+});