fix(gui-v2): mount Drawer only on mobile (v-if) + shared Tailwind breakpoint

CRITICAL: replace `lg:hidden` on PrimeVue Drawer with `v-if="isMobile"` so the
teleported portal/overlay is never created on desktop viewports regardless of
mobileOpen state. Replace useMediaQuery raw string in SidebarHeader with
useBreakpoints(breakpointsTailwind).smaller('lg') shared by both components.
Add desktop/mobile comments; adapt tests to useBreakpoints mock; add
Drawer-absent-on-desktop and aside w-16/w-64 width-class assertions (21 tests).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

apps/app/src/components-v2/layout/__tests__/SidebarHeader.spec.ts

@@ -2,7 +2,7 @@
  * SidebarHeader.spec.ts — unit tests for the non-trivial logic in SidebarHeader.
  *
  * Strategy: mount with @vue/test-utils + createPinia. Child components (Icon)
- * and @vueuse/core's useMediaQuery are stubbed so we test only:
+ * and @vueuse/core's useBreakpoints are stubbed so we test only:
  *   1. Collapse-button desktop path → shell.toggleSidebar() called.
  *   2. Collapse-button mobile path  → shell.setMobileOpen(false) called.
  *   3. Collapsed state hides wordmark + pill; keep collapse button.
@@ -18,14 +18,18 @@ import { beforeEach, describe, expect, it, vi } from 'vitest'
 import { useShellUiStore } from '@/stores/useShellUiStore'
 
 // ---------------------------------------------------------------------------
-// Mock @vueuse/core so we can control `isMobile` per test
+// Mock @vueuse/core so we can control `isMobile` per test.
+// The component uses useBreakpoints(breakpointsTailwind).smaller('lg').
+// We return an object whose .smaller() method returns a controllable ref.
 // ---------------------------------------------------------------------------
 
-// We expose a reactive ref that individual tests can flip.
 const mockIsMobileRef = ref(false)
 
 vi.mock('@vueuse/core', () => ({
-  useMediaQuery: () => mockIsMobileRef,
+  breakpointsTailwind: {},
+  useBreakpoints: () => ({
+    smaller: (_bp: string) => mockIsMobileRef,
+  }),
 }))
 
 // ---------------------------------------------------------------------------