feat: add project editor with drag & drop file management
- Backend: ProjectFileService with file CRUD, search, git status, file watcher
- IPC: 12 editor channels with security validation and path containment
- Store: editorSlice with multi-tab management, draft persistence, conflict detection
- UI: CodeMirror 6 editor, file tree with DnD, search-in-files, context menus
- Move: fs.rename with EXDEV fallback, full path remapping across all caches
- Tests: comprehensive coverage for services, IPC handlers, store, and utilities
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