T-9: Introduce @FocusedValue<WindowID> for menu command targeting

[kirich1409]

Part of EPIC #193. Depends on #201.

Description

Add a FocusedValues key currentWindowID: WindowID?. In each scene-root view, call .focusedValue(\.currentWindowID, windowID). Inside .commands { }, read via @FocusedValue(\.currentWindowID) and dispatch actions into the scoped store of the focused window.

Acceptance Criteria

• Cmd+T in window A creates a tab in window A, not in B
• Cmd+Shift+W in window A closes the project only in A
• All existing keyboard shortcuts continue to work
• When the key window is Welcome (not Main), main-only commands (New Tab, Close Project) are correctly .disabled()
• Build passes, swiftlint lint --strict clean

Module / Layer

Relay / RelayApp + scene roots

Complexity

Medium