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/gantt.module.css

@@ -0,0 +1,12 @@
+/* Gantt Chart Status Row Styles */
+.rowCompleted {
+  background-color: #f0fdf4; /* green-50 */
+}
+
+.rowInProgress {
+  background-color: #eff6ff; /* blue-50 */
+}
+
+.rowPaused {
+  background-color: #fefce8; /* yellow-50 */
+}