feat(settings): Replace static email with dynamic Supabase user email and add iOS subscription management link

[iamhenry]

Task Description

Replace the hardcoded email "zeroproof@gmail.com" in the settings screen with the dynamic email address from the authenticated Supabase user. Update the avatar to display the first letter of the user's email address. Add a FlatList section containing a single deep link item that navigates users to iOS Settings for subscription management.

Existing Context

• settings.tsx currently displays static user profile with hardcoded email "zeroproof@gmail.com" and avatar text "Z"
• useSupabase() hook already provides access to authenticated user data via user and session objects
• SupabaseProvider manages authentication state with initialization tracking
• Pro badge component is already implemented and should remain static
• Existing UI components (Avatar, Badge, Button, Text) are available for reuse

Acceptance Criteria

• Settings screen displays the authenticated user's actual email address instead of "zeroproof@gmail.com"
• Avatar shows the first letter of the user's email address (e.g., "j" for "john@example.com")
• Pro badge remains static as "Pro"
• FlatList contains exactly one item: "Manage Subscription" with deep link to iOS Settings
• Graceful handling when user email is not available (fallback display)
• No loading state required - use existing SupabaseProvider initialization

Scope and Constraints

• Modify only app/(app)/(protected)/settings.tsx
• Use existing useSupabase() hook - no new context or state management
• Leverage existing UI components from /components/ui/
• Follow KISS principle - minimal code changes
• Maintain all existing functionality (signOut, styling, layout)
• iOS-specific deep link implementation only

Dependencies

• context/supabase-provider.tsx - provides user authentication data
• components/ui/avatar.tsx - for user avatar display
• components/ui/badge.tsx - for Pro badge (unchanged)
• components/ui/button.tsx - for Sign Out button (unchanged)
• components/ui/text.tsx - for text display
• React Native Linking API for iOS Settings deep link

Resources and References

• Supabase User object documentation
• React Native Linking API for iOS Settings deep link
• iOS Settings URL schemes
• Existing SupabaseProvider implementation for user data access patterns

Deliverables

• Modified app/(app)/(protected)/settings.tsx with dynamic email display
• Updated Avatar component to show first letter of user email
• Added FlatList with iOS Settings deep link for subscription management
• Proper error handling for missing user email data

Risks or Challenges

• User email might be null/undefined during authentication state transitions
• iOS Settings deep link behavior varies across iOS versions
• Avatar logic needs proper email validation and fallback handling
• FlatList integration should maintain consistent styling with existing layout