Feature/challenge deadline notification mobile

[Mert-Kirman]

Add Challenge Deadline Display and Management

Summary

Implemented deadline functionality for challenges, allowing users to create challenges with deadlines and view deadline information with visual urgency indicators.

Changes Made

Challenge Creation

• Added deadline input field to challenge creation modal (YYYY-MM-DD format)
• Deadline is now a required field when creating challenges
• Localized "Make challenge public" toggle text

Deadline Display

• Challenges now show deadline information with clock icon (⏰)
• Smart deadline formatting:
  • "X days left" (>1 day remaining)
  • "Due tomorrow" (1 day)
  • "Due today" (0 days)
  • "Overdue by X days" (past deadline)
• Color-coded urgency indicators:
  • 🔴 Red: Overdue or ≤1 day remaining
  • 🟡 Yellow/Orange: 2-3 days remaining
  • 🟢 Green: >3 days remaining

Technical Improvements

• Added helper functions: getDaysRemaining(), formatDeadline(), getDeadlineUrgencyColor()
• Refactored UserChallenge interface and challenge filtering logic
• Improved challenge refresh functionality
• Fixed challenge enrollment state tracking

Screenshots

Challenge cards now display deadline information below the Public/Private indicator with appropriate urgency colors.

Testing

• Create a challenge with various deadline dates
• Verify deadline text formats correctly for different time ranges
• Confirm urgency colors change based on days remaining

Closes: #462

[berkayak13]

thanks for handling challenges screen + deadline
great set of changes