style: fix lint errors in test files

- Remove unused imports in config.test.ts and conversations.test.ts
- Remove unused handler variable in tauri.test.ts
- Remove unused _args parameter in vitest.setup.ts mock
- Add coverage/ to eslint ignore list

src/lib/tauri.test.ts

@@ -319,8 +319,6 @@ describe("tauri event handling", () => {
 
 describe("mock event system", () => {
   it("can emit events through mock system", () => {
-    const handler = vi.fn();
-
     // The emitMockEvent function should work
     expect(typeof emitMockEvent).toBe("function");
   });