feat: Add dark/light theme toggle button in ChatHeader

[Aryan-Verma-999]

Description

Added a runtime theme toggle button that allows users to switch between dark and light modes without refreshing.

Changes

• Created themeStore.js for theme state management
• Added sun/moon icons to ui-elements package
• Added theme toggle to ChatHeader surfaceItems
• Theme initializes from dark prop but can be toggled at runtime

Video/Screenshots

dark-mode.mp4

PR Test Details

Note: The PR will be ready for live testing at https://rocketchat.github.io/EmbeddedChat/pulls/pr-1038 after approval. Contributors are requested to replace <pr_number> with the actual PR number.

[Spiral-Memory]

Hi @Aryan-Verma-999,
Thanks for the PR, but EmbeddedChat is not meant to work like this. Developers who are integrating the app as a React component should choose a theme and provide a button to toggle the theme. This storybook is just for showcasing the project, and you can change the theme in the storybook settings. I have to close this PR as it doesn't align with what we are building. Thanks.

[Aryan-Verma-999]

Thanks for the clarification @Spiral-Memory .
Sorry about that, I misunderstood the intended usage pattern
I will stay aligned with the architecture going forward.