fix: prevent custom field identity collisions

[ArtwoXYZ]

What does this PR do?

Custom fields now use stable, server-generated IDs instead of names based on their labels.

This prevents a plugin update from accidentally creating a built-in field with the same technical name as an existing custom field.

Existing fields and values are migrated automatically on startup and after restoring a backup. Card and corner-field settings are migrated too.

Related issue

Part of #151

Type of change

• 🐛 Bug fix
• ✨ New feature
• 🧩 New or updated plugin
• 📖 Documentation
• 🌍 Translation
• 🧹 Refactor or chore

Checklist

• make typecheck passes
• I tested my changes locally
• I updated the docs or translations if needed
• My change is focused on a single thing

Screenshots

No visible UI change in this PR. The existing custom-field screens keep working as before.

Anything else?

This is the first of two PRs planned for #151.

The migration is safe to run more than once. It never overwrites an existing destination value and keeps the old values as recovery data.

Tested with:

• unit and MongoDB integration tests
• a real Docker startup migration
• instance and collection backup restores
• JSON and ZIP export/restore round trips
• editing and saving native and custom fields independently
• a manual visual check

A second PR will add the warning and conflict-resolution UI.