Automate GrowthHub CRM Social Posting

I have a working prototype of GrowthHub CRM built with plain HTML, CSS and vanilla JavaScript. It already lets users add customers and leads, manage deal stages, jot follow-up notes and plan content for Instagram, Facebook and LinkedIn, with everything stored in browser localStorage.

The next step is to turn the static social-media planner into a fully automated scheduler that can publish posts directly to those three platforms at the times users choose. That is the entire focus of this job.

Here is what I need:

• A scheduling interface that fits naturally into the existing dashboard and lets users pick date, time and target channel for each post.
• API integration that pushes the content live to Instagram, Facebook and LinkedIn (using Meta Graph API, LinkedIn API or another reliable route you recommend).
• Secure token/auth handling so users can connect their accounts once and keep posting without re-authenticating each time.
• Error handling and status feedback inside the UI (posted, queued, failed, etc.).
• Clear build/run instructions; everything should work in a standard browser environment. I am open to adding a lightweight backend if client-side alone proves impossible for the APIs.

Please send a detailed project proposal outlining your technical approach, any third-party libraries or backend stack you intend to use, milestones, and an estimated timeline. I have no strict deadline, so structure the schedule in a way that lets you deliver a polished, reliable feature set.