feat(#15): implement Gantt chart component

- Create GanttChart component with timeline visualization
- Add task bars with status-based color coding
- Implement PDA-friendly language (Target passed vs OVERDUE)
- Support task click interactions
- Comprehensive test coverage (96.18%)
- 33 tests passing (22 component + 11 helper tests)
- Fully accessible with ARIA labels and keyboard navigation
- Demo page at /demo/gantt
- Responsive design with customizable height

Technical details:
- Uses Next.js 16 + React 19 + TypeScript
- Strict typing (NO any types)
- Helper functions to convert Task to GanttTask
- Timeline calculation with automatic range detection
- Status indicators: completed, in-progress, paused, not-started

Refs #15

apps/web/src/components/gantt/index.ts

@@ -0,0 +1,7 @@
+/**
+ * Gantt Chart component exports
+ */
+
+export { GanttChart } from "./GanttChart";
+export type { GanttTask, GanttChartProps, TimelineRange, GanttBarPosition } from "./types";
+export { toGanttTask, toGanttTasks } from "./types";