test(cortex): add comprehensive tests for CortexDesktopLayout

Add 39 tests covering all major functionality of CortexDesktopLayout:

- Rendering (7): component mounts, title bar, mode carousel, notifications,
  approval dialog, children, root container styling
- Loading layout state from localStorage (10): default mode, load mode/
  sidebarTab/sidebarWidth/chatState, width clamping, invalid values,
  force IDE mode with active project
- VALID_SIDEBAR_TABS and factory handling (2): factory tab from localStorage,
  factory nav click always opens sidebar
- handleNavItemClick (5): toggle collapse on active tab, switch tab with
  sidebar open/collapsed, home switches to vibe mode, new dispatches
  file:new event
- Mode switching (5): vibe/ide layout rendering, viewmode:change event,
  folder:did-open switches to IDE and sets files sidebar
- Saving layout state to localStorage (3): mode, sidebarTab,
  sidebarCollapsed persisted on change
- Event listeners (7): registration on mount verifies all 14 events,
  cleanup on unmount uses behavioral verification (events stop being
  handled after unmount), view:explorer/search/git/extensions open
  correct sidebar tabs, sidebar:toggle toggles collapsed state

Uses waitForMount() helper for async onMount listener registration and
vi.waitFor() for reactive DOM updates. All mocks follow existing test
patterns (CortexActivityBar, CortexChatPanel).

Author: echobt