fix(shell): restore desktop sidebar state after mobile resizing

[xuzijian2019]

Fixes #1057.

Resize an open desktop sidebar to mobile, close the drawer, then resize back: the desktop sidebar stays hidden because both layouts use the same open flag.

Remember the desktop preference separately and restore it when returning to desktop. Only explicit desktop toggles update the preference; mobile drawer and toolbar actions keep their current behavior. Seed the preference from the initial URL sidebarCollapsed value so an explicitly collapsed layout stays collapsed across hydration and resizing.

Validation

• Three behavioral regression tests execute the component's real breakpoint effect and toggle callback, covering open desktop → closed mobile drawer → desktop, explicit desktop collapse → mobile open → desktop, and URL collapse with repeated breakpoint crossings.
• npm test: 2,286 tests passed.
• node_modules/.bin/tsc --noEmit and npm run lint passed.
• git diff --check passed.
• No browser E2E performed.

Adapted from the desktop fork's independent fix; original tracking: xuzijian2019/pi-agent-desktop#10. No desktop or fork UI dependencies.