feat: implement entity selection and highlighting system

[zuhanit]

Summary

• Add entity click detection and selection functionality in viewport
• Implement green selection highlight overlay with proper rendering
• Add keyboard delete functionality (Delete key) for selected entities
• Fix entity deletion to automatically clear selection when deleted entity is selected

Key Changes

• Entity Selection: Click detection with proper coordinate transformation and hit testing
• Visual Feedback: Green stroke highlight overlay for selected entities that properly appears/disappears
• Keyboard Controls: Delete key support with tabIndex focus management
• State Management: Improved entity store with proper null handling and selection clearing
• Canvas Rendering: Fixed layering and clearing issues for proper highlight display
• Transform Schema: Updated size structure from width/height to left/top/right/bottom bounds for better collision detection

Technical Improvements

• Centered unit and sprite rendering at transform positions
• Fixed canvas clearing and re-rendering pipeline
• Improved drag vs click detection in viewport interactions
• Added entity utility functions for position-based lookups
• Enhanced state synchronization between map and entity stores

Test Plan

• Test entity selection by clicking on units in viewport
• Verify green highlight appears around selected entity
• Test highlight disappears when clicking elsewhere
• Test Delete key removes selected entity and clears highlight
• Verify drag vs click distinction works properly
• Test multiple entity selection scenarios

🤖 Generated with Claude Code