fix(agents-mobile): prevent stream transition flash

[KyleAMathews]

Summary

• theme the native DOM/WebView background used by the mobile session embed
• avoid a white flash while the stream DOM bundle boots, especially in dark mode
• document that fresh worktrees need pnpm install before typecheck/tests

Verification

• pnpm install from worktree root
• pnpm run build in packages/agents-runtime
• pnpm run typecheck in packages/agents-mobile

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.41%. Comparing base (b2ddd59) to head (5013d31).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4444   +/-   ##
=======================================
  Coverage   85.41%   85.41%           
=======================================
  Files           2        2           
  Lines          48       48           
  Branches       11       11           
=======================================
  Hits           41       41           
  Misses          7        7           

Flag	Coverage Δ
packages/agents-mobile	85.41% <ø> (ø)
typescript	85.41% <ø> (ø)
unit-tests	85.41% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[github-actions]

Electric Agents Mobile Build

Android preview build for commit 5013d31.

Platform	Profile	Status	Build
Android	preview	Passed	EAS build

Workflow run