build(docker): add runtime-configured frontend image

frontend/src/api/client.ts

@@ -1,4 +1,4 @@
-const BASE = import.meta.env.VITE_BACKEND_URL ?? "http://localhost:8787";
+import { backendBaseUrl as BASE } from "./runtime-config";
 
 export async function apiFetch<T>(path: string, init?: RequestInit): Promise<T> {
   // Only declare a JSON content-type when we actually send a body. Body-less

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");
+  });
+});

frontend/src/api/runtime-config.ts

@@ -0,0 +1,17 @@
+export interface RuntimeConfig {
+  backendBaseUrl?: string;
+}
+
+declare global {
+  interface Window {
+    __THOTHII_CONFIG__?: RuntimeConfig;
+  }
+}
+
+export function resolveBackendUrl(config: RuntimeConfig | undefined): string {
+  return config?.backendBaseUrl ?? import.meta.env.VITE_BACKEND_URL ?? "";
+}
+
+export const backendBaseUrl =
+  resolveBackendUrl(typeof window === "undefined" ? undefined : window.__THOTHII_CONFIG__) ||
+  "http://localhost:8787";