T-18: Ghost project handling (folder deleted externally)

[kirich1409]

Part of EPIC #193. Depends on #196.

Description

When opening a recent project whose directory is missing / inaccessible, show an error alert "Project not found" with two options: "Remove from Recent" and "Cancel". During scene restoration, if the folder is gone, open Welcome instead of an empty project window.

Acceptance Criteria

• Missing folder → alert is shown, no crash, no empty window
• "Remove from Recent" calls projectRepository.deleteProject(path:)
• Scene restoration gracefully falls back to Welcome when canonicalPath resolves to a missing folder
• Manual test: delete a project folder in Finder → open from Recent → alert appears

Module / Layer

Relay / WelcomeFeature + MainFeature

Complexity

Medium