Feature/sponsor tier converter

[monster555]

What

• Extended the SponsorTier enum with bronze and junior levels to better categorize sponsors
• Created SponsorTierConverter to handle backend data inconsistencies using switch expressions
• Added sponsor tier display extensions following the SessionX pattern for UI logic
• Updated the sponsors view to use the new tier handling features

Why

• Ensures robust handling of varying sponsor data from the backend
• Provides consistent display of sponsor tiers throughout the app
• Improves maintainability by organizing logic in extensions
• Applies defensive programming and optimizes performance for reliability