Feature: Expose theme toggle function in window object for easy theme sync integration with client applications (Copilot Assistant)

[MateusNavarro77]

Summary

• Adds a programmatic theme toggle for the Copilot assistant and tests to validate programmatic theme changes and persistence.

Motivation

• Allow external code (or tests) to change the Copilot UI theme at runtime (Useful for Theme Sync with the client application that copilot assistant is embedded).

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Summary by cubic

Expose window.setChainlitCopilotTheme(theme) for host-app theme sync. The choice persists in vite-ui-theme and is restored on mount; the API only works while the widget is mounted; tests cover both behaviors.

^{Written for commit 3bfdc78. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 4 files

_{Re-trigger cubic}