Nextcloud Calendar

This update improves support for self-hosted and local Nextcloud instances, with new certificate trust handling, more resilient synchronization, and several fixes for recurring and Deck events. What's new: - Added support for trusting self-signed certificates directly from the app. - Added native certificate trust management using TOFU (Trust On First Use). - Added persistent certificate pinning for trusted servers. - Added automatic loading of trusted certificates when the app starts. - Added a trusted network request layer with dedicated handling for untrusted certificates. - Improved support for local Nextcloud instances using HTTPS and custom certificates. Improvements and fixes: - Improved synchronization resilience to prevent events from disappearing when a calendar fetch fails. - Fixed recurring events using an incorrect start time. - Fixed incorrect end dates for repeating events. - Fixed parsing and display of Deck calendar events. - Improved access to Nextcloud instances hosted on local networks. - Added support for user-installed certificate authorities on local HTTPS servers. - Improved network handling by preserving response content types and providing better error logging. - Fixed avatar loading and rendering. - Added several internal refactors, test improvements, and stability fixes across synchronization and networking.