fix(web): render transparent previews on white

[UtkarshUsername]

What Changed

Changed the embedded preview webview backing surface from the T3 theme background to white.

Why

Transparent top-level documents currently inherit the selected T3 application theme because the webview is composited over the app background. In dark themes this renders otherwise unstyled pages black and changes how semi-transparent page backgrounds appear.

Using a white backing surface matches the default canvas users see when opening the same page in a regular browser. Explicit page backgrounds continue to render unchanged.

UI Changes

Before:
T3 Code is on the right

After:

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes (not applicable)

Verification:

• vp fmt --check apps/web/src/browser/HostedBrowserWebview.tsx
• vp lint apps/web/src/browser/HostedBrowserWebview.tsx
• vp run --filter @t3tools/web typecheck

Created with GPT-5.6 in the Codex harness.

---

Note

Low Risk
Single visual styling change on the preview webview container with no auth, data, or routing impact.

Overview
Sets the hosted preview webview backing layer to a fixed white (bg-white) instead of the theme bg-background, so transparent or unstyled pages no longer pick up the app theme (especially dark mode) behind the guest content.

Active-tab ring/shadow and other preview chrome are unchanged; only the compositing surface behind the embedded page matches a normal browser canvas.

^{Reviewed by Cursor Bugbot for commit 7e42800. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Render transparent previews on white in HostedBrowserWebview

Changes the webview container background from the theme background color to a fixed white so transparent previews display correctly. Active-tab ring and shadow styling remain unchanged.

^{Macroscope summarized 7e42800.}

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This small, isolated Electron UI change makes transparent preview content render against hard-coded white rather than the theme background, affecting existing preview and related presentation states. Because it changes the default preview appearance, explicit human review is warranted.

^{You can add or adjust custom eligibility rules. Learn more.}