Add validated multi-scene project browser foundation

[WebCrew]

Summary

• discover and validate project-owned scene documents
• display available scenes in a dedicated project panel
• switch scenes without exposing arbitrary filesystem access
• preserve the existing unsaved-change decision flow before switching
• restore only matching validated local Scene History
• mark the initial Multi-Scene Project Browser slice complete in Phase 2

Scope

This first slice lists and opens existing top-level project scenes. Creation, rename, move and deletion remain later Phase 2 work and are not claimed here.

Local verification

• node scripts/check.cjs
• JavaScript syntax checks for main, preload and renderer
• git diff --check