Improved README with architecture, better navigation and cleaner structure!

[niyatisharma100307-crypto]

What I changed in this PR!!

🗑️ Removed

• Removed broken screenshot placeholders (Show Image boxes)
  ➕ Added
• Added architecture diagram showing how frontend, backend and database connect
• Added Prerequisites table with download links for all tools
• Added PR checklist for contributors
• Added code style guidelines for Java and React
• Added branch naming conventions (feature/, fix/, docs/, etc.)
• Added "stay up to date with upstream" step in contribution guide
• Added color-coded difficulty table for open issues

🔧 Fixed

• Fixed broken TOC links for Tech Stack and Architecture sections
  (caused by special emoji characters that GitHub couldn't read)
• Replaced 🏗️ with 🔧 and 🏛️ with 📐 to fix anchor links

♻️ Improved

• Rewrote Quick Start with clear numbered steps
• Converted Tech Stack from bullet points to tables (shows version + role)
• Split Features into three clear sections: Available / In Progress / Roadmap
• Converted Troubleshooting into collapsible dropdowns (cleaner look)
• Improved Contact section into a clean table layout
• Added 2 extra badges (PostgreSQL, Gradle) to the header

📄 Type of change

• Documentation only (no code was changed)

closes #137