EXT-3: Migrate Custom Themes to Extension Format

[ZhuchkaTriplesix]

Epic: #206
Depends on: EXT-2

Goal

Migrate the existing custom theme system to use the new extension format.

Implementation

• Treat UI themes as extensions with type: "theme".
• Refactor the current custom theme loader to use LocalExtensionRegistry.
• Update the Theme Picker UI to load themes from the unified extension registry.