feat: Major UX improvements for mobile responsiveness and authentication

[algsoch]

🎯 Authentication & Signup Improvements

• ✅ Fixed login to work with both username AND email
• ✅ Added username availability check (real-time validation)
• ✅ Enhanced error messages for login/signup failures
• ✅ Added loading states and success feedback
• ✅ Improved signup flow with better validation
• ✅ Added comprehensive error handling

📱 Mobile UX Enhancements

• ✅ Fixed bottom navigation overlap issues
• ✅ Made all modals mobile-responsive
• ✅ Centered settings modal for better visibility
• ✅ Fixed sign out button visibility on mobile
• ✅ Improved toast notifications (4s duration, z-index 9999)
• ✅ Enhanced username availability feedback (color-coded borders)

👤 Profile Management

• ✅ Implemented full profile editing (name, username, DOB, gender, occupation, location)
• ✅ Added avatar upload preview functionality
• ✅ Real-time username availability checking
• ✅ Profile settings modal with scrollable content
• ✅ Added camera icon for profile picture upload

🎯 Habit Settings

• ✅ Made create habit modal fully responsive
• ✅ Fixed button visibility issues on mobile
• ✅ Improved scrolling and layout
• ✅ Added navigation from profile settings to habits

🔧 Backend Improvements

• ✅ Fixed username-to-email lookup for login
• ✅ Made username availability endpoint public (no auth required)
• ✅ Added friend suggestions endpoint
• ✅ Enhanced error logging and debugging
• ✅ Better error messages for authentication

🎨 UI/UX Polish

• ✅ Enhanced Toast component (responsive sizing, longer duration)
• ✅ Color-coded username status (green for available, red for taken)
• ✅ Removed empty spaces in signup form
• ✅ Consistent button sizing and spacing
• ✅ Better visual feedback for all user actions

🐛 Bug Fixes

• ✅ Fixed settings modal cut-off issues on mobile
• ✅ Fixed username availability not showing
• ✅ Fixed login errors not displaying
• ✅ Fixed modal height issues on small screens
• ✅ Fixed button overlap with bottom navigation