test(frontend/copilot): cover localStorage initialiser branches with resetModules

The copilotChatMode and copilotLlmModel store fields are initialised from
localStorage via IIFEs that run once at module-load time. The existing tests
reset state with setState() which bypasses the initialiser, leaving the 'fast'
and 'advanced' branches uncovered for codecov patch.

Use vi.resetModules() + dynamic import to get a fresh store instance with
pre-populated localStorage, covering both initialiser branches.

autogpt_platform/frontend/src/app/(platform)/copilot/__tests__/store.test.ts

@@ -1,4 +1,4 @@
-import { describe, expect, it, beforeEach, vi } from "vitest";
+import { describe, expect, it, beforeEach, afterEach, vi } from "vitest";
 import { useCopilotUIStore } from "../store";
 
 vi.mock("@sentry/nextjs", () => ({
@@ -243,3 +243,24 @@ describe("useCopilotUIStore", () => {
     });
   });
 });
+
+describe("useCopilotUIStore localStorage initialisation", () => {
+  afterEach(() => {
+    vi.resetModules();
+    window.localStorage.clear();
+  });
+
+  it("reads fast chat mode from localStorage on store creation", async () => {
+    window.localStorage.setItem("copilot-mode", "fast");
+    vi.resetModules();
+    const { useCopilotUIStore: fresh } = await import("../store");
+    expect(fresh.getState().copilotChatMode).toBe("fast");
+  });
+
+  it("reads advanced model from localStorage on store creation", async () => {
+    window.localStorage.setItem("copilot-model", "advanced");
+    vi.resetModules();
+    const { useCopilotUIStore: fresh } = await import("../store");
+    expect(fresh.getState().copilotLlmModel).toBe("advanced");
+  });
+});