Add UI controls in post editor for rewriting titles

[jeffpaul]

Description

Provide basic UI controls within the post editor for Title Rewriting functionality (see more in #10). The feature should follow the plugin’s encapsulation model so it can operate as an independent module within the broader plugin structure.

Goals

• Add contextual UI controls in the post editor (for example, a “Rewrite Title” button near the post title input).
• Load UI components and logic only if the Title Rewriting feature is enabled and experimental features are globally active.
• Implement this as a self-contained feature module with its own scripts, styles, and PHP logic within /features/title-rewriting/.
• Ensure UI communicates clearly when no provider is configured.
• Support graceful fallback behavior and clear user feedback.

Acceptance Criteria

• UI appears only when experimental features are enabled.
• Title rewriting flow works end-to-end using the Abilities API and AI Client SDKs.
• Feature operates independently and can be reasonably replicated into a different codebase without major refactoring.

[jeffpaul]

Closing as complete with #61, #67, and #83.