feat(phase-13): implement gradient tool with linear, radial, and angular modes

Add comprehensive gradient tool with three gradient types and full UI integration.

Features:
- Gradient tool with drag-to-create interaction
- Three gradient types: Linear, Radial, and Angular (conic)
- Live preview during drag with 70% opacity overlay
- Primary and secondary color selection
- Gradient type selector in tool options
- Undo/redo support through command system
- Fallback to radial gradient for browsers without conic gradient support

Changes:
- Created tools/gradient-tool.ts with GradientTool class
- Added 'gradient' to ToolType in types/tool.ts
- Extended ToolSettings with secondaryColor and gradientType
- Updated store/tool-store.ts with setSecondaryColor and setGradientType methods
- Added gradient tool loading in lib/tool-loader.ts
- Added gradient button to tool palette with 'G' shortcut
- Added gradient tool options UI in components/editor/tool-options.tsx

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

types/tool.ts

@@ -10,6 +10,7 @@ export type ToolType =
   | 'brush'
   | 'eraser'
   | 'fill'
+  | 'gradient'
   | 'eyedropper'
   | 'text'
   | 'shape'
@@ -32,6 +33,10 @@ export interface ToolSettings {
   hardness: number;
   /** Color */
   color: string;
+  /** Secondary color (for gradients) */
+  secondaryColor?: string;
+  /** Gradient type */
+  gradientType?: 'linear' | 'radial' | 'angular';
   /** Flow rate (0-1) */
   flow: number;
   /** Spacing between brush stamps */