Add filter for age limit in Gamevault client

[strese]

Is your feature request related to a problem? Please describe.
The ability to filter games based on their age rating would be a valuable feature. Potential use cases include:

• Filtering games suitable for a specific age group.
• Identifying games without any populated age rating, such as in cases where the rating is missing from the IGDB page.
  This functionality would enhance library organization and make it easier to detect incomplete metadata.

Describe the solution you'd like
Introduce a filter option for age rating, similar to existing filters like "Game Type," "Tags," and "Genres." This filter would use a dropdown menu displaying the available age rating values found within the user's library.
Games without a defined age rating is displayed with N/A in their profile in Gamevault. If such games exist in the library, N/A should appear in the dropdown menu as an option. This will help users quickly identify any titles that lack an age rating, making it easier to spot missing data.

Describe alternatives you've considered
Without a dedicated age rating filter, the only alternative is to manually review each game in the library one by one. This process is required to identify titles that lack a populated age rating, and then update them individually—resulting in a time-consuming and inefficient workflow