feat(ai): build opt-in model setup and lifecycle controls

[McanKul]

Add the settings flow used to install, import, load, unload, and remove the optional local model.

Depends on the verified artifact store and hardware compatibility checks.

Acceptance criteria

• The UI shows model size, license, location, and compatibility before setup.
• Download or import requires an explicit user action.
• Loading and inference can be cancelled.
• Removal confirms the target and reports recovered disk space.
• Restarting the app reflects the real model state.
• OffPDF remains fully usable when no model is installed.

[McanKul]

The offline artifact store, lifecycle commands, cancellation/removal behavior, and prepared Settings components landed via #107. The user-facing Settings entry remains deliberately hidden until the real runtime and #82 hardware compatibility check are connected, so this issue stays open for the actual product completion.