Update CONTRIBUTING.md with environment setup details

[ishitaajain22-tech]

Description

Added clear instructions in CONTRIBUTING.md about renaming .env.example → .env.local and avoiding spaces after = in API keys. This prevents the common "Missing Supabase env vars" error.
Fixes common setup issues for new contributors.

Fixes #397

Type of Change

• Documentation update (README, contribution guidelines, etc.)

Changes Made

• "Rename .env.example → .env.local (required for Next.js dev)"
• "No spaces after =: KEY=value, not KEY= value"

Dependencies

• None

Add Screenshots

N/A (docs only)

Checklist

• My code follows the style guidelines of this project.
• I have tested my changes in development mode (npm run dev)
• This is already assigned Issue to me, not an unassigned issue.

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[Muneerali199]

push to dev as it is primary branch for contributing and main branch is for publishing

[ishitaajain22-tech]

@Muneerali199 done