feat(ui): add relationship management features in schema editor

- Implement functionality to create, update, delete, and view relationships between tables.
- Introduce new UI screens for managing relationships, including forms for adding and editing relationships.
- Enhance table editor with navigation to relationship management.
- Ensure relationships are displayed in a structured table format for better usability.

pkg/ui/doc.go

@@ -0,0 +1,57 @@
+// Package ui provides an interactive terminal user interface (TUI) for editing database schemas.
+//
+// # Overview
+//
+// The ui package implements a full-featured terminal-based schema editor using tview,
+// allowing users to visually create, modify, and manage database schemas without writing
+// code or SQL.
+//
+// # Features
+//
+// The schema editor supports:
+//   - Database management: Edit name, description, and properties
+//   - Schema management: Create, edit, delete schemas
+//   - Table management: Create, edit, delete tables
+//   - Column management: Add, modify, delete columns with full property support
+//   - Relationship management: Define and edit table relationships
+//   - Domain management: Organize tables into logical domains
+//   - Import & merge: Combine schemas from multiple sources
+//   - Save: Export to any supported format
+//
+// # Architecture
+//
+// The package is organized into several components:
+//   - editor.go: Main editor and application lifecycle
+//   - *_screens.go: UI screens for each entity type
+//   - *_dataops.go: Business logic and data operations
+//   - dialogs.go: Reusable dialog components
+//   - load_save_screens.go: File I/O and format selection
+//   - main_menu.go: Primary navigation menu
+//
+// # Usage
+//
+//	editor := ui.NewSchemaEditor(database)
+//	if err := editor.Run(); err != nil {
+//	    log.Fatal(err)
+//	}
+//
+// Or with pre-configured load/save options:
+//
+//	editor := ui.NewSchemaEditorWithConfigs(database, loadConfig, saveConfig)
+//	if err := editor.Run(); err != nil {
+//	    log.Fatal(err)
+//	}
+//
+// # Navigation
+//
+//   - Arrow keys: Navigate between items
+//   - Enter: Select/edit item
+//   - Tab/Shift+Tab: Navigate between buttons
+//   - Escape: Go back/cancel
+//   - Letter shortcuts: Quick actions (e.g., 'n' for new, 'e' for edit, 'd' for delete)
+//
+// # Integration
+//
+// The editor integrates with all readers and writers, supporting load/save operations
+// for any format supported by RelSpec (DBML, PostgreSQL, GORM, Prisma, etc.).
+package ui