fix(tools): harden scheduler limits and logs

Guard scheduler system actions against pending work, serialize diagnostic log transitions, and keep speed-limit saves truthful after backend failures.\n\nNo linked issue was found for this audit.

src/store/useSettingsStore.test.ts

@@ -0,0 +1,27 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import { useSettingsStore } from './useSettingsStore';
+import * as ipc from '../ipc';
+
+vi.mock('../ipc', () => ({
+  invokeCommand: vi.fn()
+}));
+
+vi.mock('../utils/logger', () => ({
+  info: vi.fn()
+}));
+
+describe('useSettingsStore global speed limit persistence', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    useSettingsStore.setState({ globalSpeedLimit: '2M' });
+  });
+
+  it('keeps the saved value when the backend rejects a limit change', async () => {
+    vi.mocked(ipc.invokeCommand).mockRejectedValueOnce(new Error('aria2 unavailable'));
+
+    await expect(useSettingsStore.getState().setGlobalSpeedLimit('3M')).rejects.toThrow('aria2 unavailable');
+
+    expect(useSettingsStore.getState().globalSpeedLimit).toBe('2M');
+    expect(ipc.invokeCommand).toHaveBeenCalledWith('set_global_speed_limit', { limit: '3M' });
+  });
+});