feat: implement dark mode logic, demo content, and brand assets

[trivedikavya]

Description:
This PR is related to #592 In This PR transitions the Community Forum from a static "loading" state to a fully interactive and themed user interface. It focuses on enhancing user experience through persistent theme settings and realistic data visualization.

Key Changes:

• Theme Engine: Integrated JavaScript logic to toggle between Light and Dark modes. Added localStorage support to persist user theme preferences across sessions.
• Demo Feed Implementation: Replaced the "Loading conversations" placeholder with high-fidelity demo cards (Crop Protection, Soil Health, and Equipment Advice) to showcase the UI's content layout.
• Accessibility & UX: Standardized the favicon across the forum for brand consistency.
• Refined CSS variables for better contrast ratios in Dark Mode.
• Added a smooth "Back to Top" interaction for long-form discussion feeds.
• Bug Fixes: Resolved an issue where theme classes weren't correctly applied to the navbar and modal components.

---

@omroy07 Sir This below issues are completed You can Close this Issues Sir

closes #104
closes #121
closes #35

[vercel]

@trivedikavya is attempting to deploy a commit to the Om Roy's projects Team on Vercel.

A member of the Team first needs to authorize it.

[github-actions]

Thanks for creating a PR for your Issue! ☺️

We'll review it as soon as possible.
In the meantime, please double-check the file changes and ensure that all commits are accurate.

If there are any unresolved review comments, feel free to resolve them. 🙌🏼

[github-actions]

🎉 Congrats @trivedikavya on getting your PR merged! 🙌
Thanks for the contribution — looking forward to more from you 🚀