fix(http): preserve SDK headers when injecting User-Agent

The v1.76.0 User-Agent injection object-spread `fetchOptions.headers`, which
silently drops all headers when the value is a Headers instance. The Immich
SDK passes a Headers object with the merged x-api-key header, so requests
lost their API key and returned 401 (issue #6672). Build a real Headers
instance instead, keeping case-insensitive user-agent handling.

packages/core/src/test/infrastructure/http/request.spec.ts

@@ -0,0 +1,50 @@
+import { describe, expect, test } from "vitest";
+
+import { mergeHeadersWithUserAgent } from "@homarr/core/infrastructure/http";
+
+describe("mergeHeadersWithUserAgent", () => {
+  test("should keep existing headers when given a Headers instance", () => {
+    // Arrange
+    const headers = new Headers({ "x-api-key": "some-api-key", Accept: "application/json" });
+
+    // Act
+    const merged = mergeHeadersWithUserAgent(headers);
+
+    // Assert
+    expect(merged.get("x-api-key")).toBe("some-api-key");
+    expect(merged.get("accept")).toBe("application/json");
+    expect(merged.get("user-agent")).toContain("Homarr/");
+  });
+
+  test("should keep existing headers when given a record", () => {
+    // Arrange
+    const headers = { "x-api-key": "some-api-key", Accept: "application/json" };
+
+    // Act
+    const merged = mergeHeadersWithUserAgent(headers);
+
+    // Assert
+    expect(merged.get("x-api-key")).toBe("some-api-key");
+    expect(merged.get("accept")).toBe("application/json");
+    expect(merged.get("user-agent")).toContain("Homarr/");
+  });
+
+  test("should not override an existing user-agent in any casing", () => {
+    // Arrange
+    const headers = new Headers({ "USER-AGENT": "custom-agent" });
+
+    // Act
+    const merged = mergeHeadersWithUserAgent(headers);
+
+    // Assert
+    expect(merged.get("user-agent")).toBe("custom-agent");
+  });
+
+  test("should fall back to the default user agent when no headers are provided", () => {
+    // Act
+    const merged = mergeHeadersWithUserAgent(undefined);
+
+    // Assert
+    expect(merged.get("user-agent")).toContain("Homarr/");
+  });
+});