mweinbach4/30/2026, 2:47:52 PM

Move design-invariant titlebar/drag-strip rules to a shared platform
stylesheet and replace hardcoded pixel values in darwin/win32/linux
overrides with CSS variables fed by the platform chrome contract.

- Add styles/platform/shared.css that pulls .app-topbar--frame height,
  .app-window-drag-strip height, and the CSS-blur disable toggle from
  --platform-* variables set by App.tsx.
- Replace 48px and 2.375rem magic numbers in darwin/win32/linux CSS
  with var(--platform-titlebar-height) and caption reserves with
  var(--platform-caption-button-reserve).
- Add docs/platform-chrome.md describing the contract, the IPC wiring,
  the renderer hook, and how to add new platform tweaks.

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