Migrate `ThemeModel` to `Room`

[wzieba]

Related: #15011 Closes: AINFRA-548: Migrate ThemeModel

Description

This PR migrates ThemeModel from WellSql to Room. As always, these PRs focus on migrations, but in this specific case I also decided to migrate a few classes from Java to Kotlin, to be able to run suspend methods.

Test Steps

Smoke test theme functionality:

1. Open the app and navigate to Settings → Site Settings → Themes
2. Verify the current theme is displayed correctly
3. Browse available themes and verify they load with names and preview images
4. Select a different theme and activate it
5. Verify the theme activation succeeds and the new theme is shown as active

Images/gif

Before (trunk)

After

• [ ] I have considered if this change warrants release notes and have added them to RELEASE-NOTES.txt if necessary. Use the "[Internal]" label for non-user-facing changes.