Refactor GridManager with new DateColumnDataSource

Introduces DateColumnDataSource to centralize date column generation logic
Simplifies GridManager by delegating date calculations to dedicated data source
Enhances flexibility for different calendar views and date rendering strategies

Improves separation of concerns and makes calendar view management more modular

src/types/ColumnDataSource.ts

@@ -0,0 +1,18 @@
+/**
+ * IColumnDataSource - Defines the contract for providing column data
+ *
+ * This interface abstracts away whether columns represent dates or resources,
+ * allowing the calendar to switch between date-based and resource-based views.
+ */
+export interface IColumnDataSource {
+  /**
+   * Get the list of column identifiers to render
+   * @returns Array of identifiers (dates or resource IDs)
+   */
+  getColumns(): Date[];
+
+  /**
+   * Get the type of columns this datasource provides
+   */
+  getType(): 'date' | 'resource';
+}