mweinbach4/30/2026, 2:47:45 PM

Split native topbar chrome into a dedicated component so the shared
AppTopBar no longer branches on raw process.platform strings.

- New PlatformTopBarChrome component renders the macOS
  SidebarCollapseControl, Windows collapsed left rail, or Linux/other
  inline toggle based on topbarControlPlacement from the contract.
- AppTopBar now uses useDesktopPlatform() instead of reading
  document.documentElement.dataset.platform inline.
- Sidebar reads sidebarTitlebandMode === "native" from the hook to
  decide whether to activate Windows-style native titleband drag zones.
- Add platform-top-bar-chrome.test.tsx covering all three placements.

No visual changes; class names and existing topbar/sidebar tests are
preserved so the existing UI contracts still hold.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

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