Draft: Add optional local due reminders

[default-student]

Context

This is a draft/reference PR for an optional local due-reminder implementation. I understand this may not match the project's product philosophy or maintenance direction, so I am opening it as a draft rather than asking for immediate merge consideration.

The main goal is to make the patch visible for people who want local device reminders without google play or extra server setup.

The code is fully generated, a working version and poc was the only goal!

What this patch does

• Adds an optional setting for local due reminders, circumventing the need for the nextcloud files app and setting up server notifications there. It works without google play.
• Schedules local reminders for cards with due dates after sync/database updates.
• Reschedules reminders after reboot/package replacement/time changes.
• Shows a local reminder notification/fullscreen reminder activity.
• Includes snooze support from the reminder UI.

Notes

• This branch currently targets the F-Droid/release application id, not the dev flavor.
• A signed fork build is available here for users who want to test the behavior: https://github.com/default-student/nextcloud-deck/releases/tag/v1.25.5-local-reminders.1
• I am happy for this to remain a reference PR if upstream does not want to carry local reminder behavior.

Testing

• Built app:assembleFdroidRelease successfully.
• Verified the release APK signature locally with apksigner verify.
• Full automated test suite has not been run yet.

Thank you for your Software!