DRAFT feat(settings): sync visual preferences across accounts

[jesperordrup]

This is a draft.

You asked for input :-)

Relates to #618

Todo:

• Test more

• Add Missing a visual option to enable/disable - but i didnt know where to put it

  When normal Settings Sync is enabled, visual preferences are now shared across all logged-in accounts on the same
  instance.

  Behaviour

  • Saving settings in any account mirrors its visual preferences to every other account.
  • The most recently saved visual preferences win.
  • Newly added accounts without a settings record are seeded with the existing visual preferences.
  • Account-specific settings remain independent.

  Mirrored preferences include theme, locale, density, font size, mail layout/list display, sidebar appearance,
  toolbar preferences, attachment display, and related presentation settings.

  Notes

  • This reuses the existing per-account settings API and storage.
  • No new backend storage model or UI is introduced.