Update README.md

[Nitya-003]

🔗 Related Issue

As proposed in issue #641 , this PR adds live-rendered Mermaid.js diagrams to the README.md. These diagrams help contributors visualize the complex logic flow between the Streamlit UI, the Gemini API, and our SQLite database.

• Closes Add Mermaid.js system architecture and data flow diagrams #641

What's Included:

1. High-Level Flow (Sequence Diagram): Details the lifecycle of a user message and how the "Tone Selector" influences the Gemini prompt.
2. Feature Integration (Flowchart): Shows the conditional routing based on user mood (Yoga vs. Coping Tips vs. Crisis Page).

Why this is needed:

• Improves developer onboarding for SWOC'26.
• Provides architectural transparency for the "TalkHeal" logic.

[Adii0906]

hey @Nitya-003 do follow on linkdin: https://www.linkedin.com/in/aditya-j-458824313