Add Twitter/X OAuth authentication for login and account linking

[adityajha2005]

Summary

Implements Twitter/X OAuth authentication allowing users to:

• Connect their X account from the profile page
• Sign in using their X account on the login page
• Automatically link X accounts to existing users by email

Changes Made

• Added OAuth controller with callback and disconnect actions
• Added twitter_uid field to User model with uniqueness validation
• Added OAuth routes for Twitter authentication
• Updated profile page with Connect/Disconnect X buttons
• Updated login page with "Sign in with X" option
• Added comprehensive test coverage (5 test files)

Files Modified (6)

• Gemfile - OAuth gems
• config/routes.rb - OAuth routes
• app/models/user.rb - twitter_uid validation
• Profile & login views - OAuth buttons
• README.md - Setup instructions

Files Added (5)

• OAuth controller and initializers
• Database migration for twitter_uid
• Comprehensive test suite

Testing

• All OAuth flows tested (connect, login, disconnect, edge cases)
• Maintains existing functionality

Resolves #20