fix(deploy): unify backend URL policy

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

@@ -1,6 +1,7 @@
 import { describe, expect, it } from "vitest";
 
 import { backendBaseUrl, joinBackendPath, resolveBackendUrl } from "./runtime-config";
+import cases from "./backend-url-cases.json";
 
 describe("resolveBackendUrl", () => {
   it("uses the runtime-injected backend URL", () => {
@@ -28,6 +29,12 @@ describe("resolveBackendUrl", () => {
       expect(resolveBackendUrl({ backendBaseUrl })).toBe(backendBaseUrl);
     },
   );
+
+  it.each(cases)("applies the canonical policy to $value", ({ value, valid }) => {
+    const resolve = () => resolveBackendUrl({ backendBaseUrl: value });
+    if (valid) expect(resolve()).toBe(value);
+    else expect(resolve).toThrow(/BACKEND_BASE_URL/);
+  });
 });
 
 describe("joinBackendPath", () => {