feat: pick (absolute) date/time for reminders

[nofishonfriday]

ref: #214
Currently this is a minimal (but working) implementation just to check if I'm on the right path (feedback welcome).
Likely needs a good bit of polishing (e.g. check that user doesn't set a reminder after an event etc.).

Needs my same named Commons branch merged too to build:
https://github.com/nofishonfriday/Fossify-Commons/tree/feat-pick-date-time-for-reminder

Type of change(s)

• Bug fix
• Feature / enhancement
• Infrastructure / tooling (CI, build, deps, tests)
• Documentation

What changed and why

• Added an option to choose (absolute) date/time for reminders via a date/time picker

Tests performed

• Basic manual test in simulator

Before & after preview

After: (note new option: "Pick date and time")

Closes the following issue(s)

• Closes Specify exact date/time for reminder #214

Checklist

• I read the contribution guidelines.
• I manually tested my changes on device/emulator (if applicable).
• I updated the "Unreleased" section in CHANGELOG.md (if applicable).
• I have self-reviewed my pull request (no typos, formatting errors, etc.).
• I understand every change in this pull request.