fix(titlebar): Clear macOS traffic lights via window controls overlay

[charlesvien]

Problem

On macOS Tahoe the traffic lights are bigger and wider than on earlier versions, so the title bar's hardcoded 78px left inset no longer clears them and the PostHog logo renders under the window controls.

Before:

After:

Changes

• Set titleBarOverlay: true on the macOS window, which enables the Window Controls Overlay CSS env vars (titlebar-area-*) reporting the system bounds of the control strip.
• The title bar's left padding on macOS is now env(titlebar-area-x, 78px) instead of pl-[78px], so it tracks whatever the running macOS version needs (88px on Tahoe). Other platforms keep the fixed 78px.

How did you test this?

• Scratch Electron window on macOS Tahoe with the production window config: env(titlebar-area-x) reports 88px and content padded with it starts exactly at the system-reported edge, clear of the lights.
• pnpm typecheck and Biome pass.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit d5a8266.}

[greptile-apps]

_{Reviews (1): Last reviewed commit: "remove pr screenshot" | Re-trigger Greptile}

[github-actions]

Low-risk titlebar layout fix: enables the Window Controls Overlay API in the Electron main process to expose CSS env vars, then uses env(titlebar-area-x, 78px) with a safe hardcoded fallback in the renderer. Both changes are in the correct architectural layers and the fallback prevents any regression if the env var isn't available.