Improve Timezone Handling and Fix End Date Picker

[a-mnich]

This pull request introduces several changes to the Jellyfin.Plugin.PlaybackReporting plugin, primarily focusing on handling user-server timezone offsets more accurately and fixing the end date date picker across various reports.

Timezone Handling Improvements

• Added a new method, CalculateUserServerTimezoneOffset, to compute the timezone offset between the user and the server. This is necessary because the plugin currently saves playback reporting database entries using the server's local timezone (instead of UTC). Therefore, a timezone offset is only needed if the timezones of the user and server differ.
• Although saving the entries directly in UTC would be the cleaner approach, changing this now would be a breaking change for all data already saved. I wanted to avoid this.
• Updated the GetUsageForUser, GetUsageForDays, GetHourlyUsageReport, GetBreakdownReport, GetTvShowReport, GetMoviesReport, and GetUserReport methods to use the new timezone offset calculation method.

Invalid URL Parameter

Fixed the URL parameter end_date/endDate in various JavaScript API calls, which caused the end date date picker not to work.

resolves #47
resolves #31
resolves #94