fix: address code review feedback

- Replace type assertions with type guards in types.ts (isDateString, isStringArray)
- Add useCallback for event handlers (handleTaskClick, handleKeyDown)
- Replace styled-jsx with CSS modules (gantt.module.css)
- Update tests to use CSS module class name patterns

apps/web/src/components/gantt/GanttChart.test.tsx

@@ -91,7 +91,7 @@ describe("GanttChart", () => {
       render(<GanttChart tasks={tasks} />);
       
       const taskRow = screen.getAllByText("Completed Task")[0].closest("[role='row']");
-      expect(taskRow).toHaveClass(/completed/i);
+      expect(taskRow?.className).toMatch(/Completed/i);
     });
 
     it("should visually distinguish in-progress tasks", () => {
@@ -106,7 +106,7 @@ describe("GanttChart", () => {
       render(<GanttChart tasks={tasks} />);
       
       const taskRow = screen.getAllByText("Active Task")[0].closest("[role='row']");
-      expect(taskRow).toHaveClass(/in-progress/i);
+      expect(taskRow?.className).toMatch(/InProgress/i);
     });
   });