Add blur tool to annotation editor

[morvader]

Summary

This PR adds a new blur annotation tool to the annotation editor, allowing users to obscure sensitive regions of images by applying a blur effect to selected rectangular areas.

Key Changes

• New blur tool implementation: Added drawBlur() function that uses canvas clipping and CSS filters to apply a configurable blur effect to selected regions
• Blur radius constant: Introduced BLUR_RADIUS = 12px to control the strength of the blur effect
• Tool integration:
  • Added blur tool to the drawing pipeline (mouse move and mouse up handlers)
  • Integrated blur annotations into the redraw/replay system
  • Added blur tool button to the UI with custom cursor styling
• Keyboard shortcut: Added 'B' key binding to activate the blur tool
• UI updates:
  • New blur tool button with custom SVG icon
  • Active state styling with indigo color scheme (#4F46E5)
  • Custom cursor with dashed circle indicator
  • Updated keyboard shortcuts help text

Implementation Details

• The blur effect is implemented using canvas ctx.clip() to confine the blur to the selected rectangle, preventing it from bleeding outside the bounds
• The blur is applied by redrawing the base image with ctx.filter = 'blur()' within the clipped region
• Blur annotations are stored with type, position (x, y), and dimensions (width, height)
• Minimum size validation (2px) prevents accidental blur creation from small mouse movements
• Supports negative width/height values (when dragging right-to-left or bottom-to-top) by normalizing rectangle coordinates

https://claude.ai/code/session_01TbfSaMsdyScyRiY8m1xYrd