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>

lib/tool-loader.ts

@@ -47,6 +47,11 @@ async function loadTool(toolKey: string): Promise<BaseTool> {
         tool = new FillTool();
         break;
       }
+      case 'gradient': {
+        const { GradientTool } = await import('@/tools/gradient-tool');
+        tool = new GradientTool();
+        break;
+      }
       case 'eyedropper': {
         const { EyedropperTool } = await import('@/tools/eyedropper-tool');
         tool = new EyedropperTool();