Fix Windows thread edge background

[SunkenInTime]

Summary

• align the dark pre-React document surface with the native Windows/workspace background
• derive browser chrome and the Windows resize inset from the stable --app-chrome-background token instead of whichever route surface happens to be mounted

Root cause

Windows reserves a six-pixel resize inset at the right edge of the frameless window. The document background behind that inset was synchronized from main or the sidebar. During a thread transition, those route surfaces can be absent or expose a different color, leaving the inset darker than the workspace. The initial dark document color also used the older #161616 value while the current workspace/native window uses #0a0a0a.

Windows screenshots

Captured from the same isolated Windows fixture and window bounds after switching from a short thread to a long, scrollable thread.

Before (nightly baseline)	After

In the fixed build, the workspace and the full six-pixel right inset both sample as rgb(10, 10, 10) above and below the scrollbar.

Verification

• targeted vp fmt --check and vp lint for the changed files
• pnpm --filter @t3tools/web typecheck
• native Windows Electron pass using an isolated state directory, switching between short and long scrollable threads

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