Add justfile and essential recipes. Add unit tests. Add linter and fix linter errors.

ui/src/api/url.test.ts

@@ -0,0 +1,16 @@
+import { describe, expect, it } from "vitest";
+import { buildApiUrl } from "./url";
+
+describe("buildApiUrl", () => {
+  it("joins base url and relative path without duplicate slashes", () => {
+    expect(buildApiUrl("/lok/open-hours")).toBe(
+      "http://localhost:5013/lok/open-hours",
+    );
+  });
+
+  it("accepts path without leading slash", () => {
+    expect(buildApiUrl("lok/open-hours/1")).toBe(
+      "http://localhost:5013/lok/open-hours/1",
+    );
+  });
+});