feat: visible date range

Signed-off-by: Sebastian Krupinski <krupinski01@gmail.com>

src/types/index.ts

@@ -7,6 +7,12 @@
 // Local UI-specific types
 export type CalendarView = 'days' | 'month' | 'agenda';
 
+/** Half-open local calendar range: [startMs, endMs). */
+export interface VisibleDateRange {
+  startMs: number;
+  endMs: number;
+}
+
 export interface ViewState {
   currentView: CalendarView;
   currentDate: Date;