T-16: Multi-monitor positioning defaults

[kirich1409]

Part of EPIC #193. Depends on #201.

Description

A new project window opens positioned on the key window's screen, or cascaded from the key window (50pt down, 30pt right). Handle monitor disconnect gracefully — existing windows should reappear on the main screen when their original monitor is unplugged.

Acceptance Criteria

• .defaultPosition(.center) (or a computed cascade offset) is applied to the project WindowGroup
• Manual test on a multi-monitor setup: a new window appears near the currently active window
• Disconnecting a monitor does not orphan windows — they reappear on the main screen

Module / Layer

Relay / RelayApp

Complexity

Small