Add desktop pet overlay with background activity

[Emanuele-web04]

Summary

• Adds a new always-on-top desktop pet overlay window with drag, move, close, and visibility controls in the Electron main process.
• Bridges pet overlay state through preload IPC and main-process event handling so the UI can render and update the pet from app state.
• Introduces web UI support for background thread activity, including a dock, pet model/animation logic, and activity-aware presentation.
• Wires app focus/blur/minimize behavior so the pet can appear when the app is backgrounded and stay detached from normal app focus.
• Expands contracts and tests around pet state, overlay behavior, and orchestration wiring.

Testing

• Not run (no verification commands were requested in this turn).
• Existing and new unit tests were added/updated for pet overlay, background activity, and thread activation flows.