Fix issue903

[JSE19]

Description
Adds frontend/.env.example documenting all 11 NEXT_PUBLIC_* environment variables the frontend reads, with comments and testnet defaults. Expands the environment variable table in docs/ARCHITECTURE.md from 4 to 11 entries. Updates the frontend setup section in CONTRIBUTING.md to reference the new .env.example file.

Type of Change

• 📚 Documentation update
  Related Issues
  Closes [Docs] Add frontend/.env.example documenting NEXT_PUBLIC_* variables #903

Changes Made

• Created frontend/.env.example listing all 11 NEXT_PUBLIC_* vars with grouped comments and testnet defaults
• Expanded frontend env table in docs/ARCHITECTURE.md:230-237 from 4 rows to all 11 vars
• Updated CONTRIBUTING.md:206-207 to reference frontend/.env.example instead of generic "create .env.local if needed"
• Added !.env.example negation to frontend/.gitignore so the file is trackable (was hidden by .env* pattern)
  Checklist
• My code follows the project's style guidelines
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• My changes generate no new warnings