fix: detect tasks created by external CalDAV clients (Thunderbird)

Tasks created in Thunderbird via CalDAV were not visible because
getCalendarTasks() used a strict @type === 'Task' check. Stalwart
may not set @type when converting VTODO from CalDAV to JMAP.

- Use case-insensitive @type matching for server variations
- Add fallback heuristic: detect tasks by presence of 'progress'
  property (exclusive to JSCalendar Task, never on Event objects)
- Normalize @type to 'Task' on detected tasks for consistent
  downstream handling
- Refresh task store on CalendarEvent state changes so tasks
  created externally appear without manual page refresh

Fixes #84

stores/email-store.ts

@@ -1102,6 +1102,12 @@ export const useEmailStore = create<EmailStore>((set, get) => ({
           if (dateRange && selectedCalendarIds.length > 0) {
             calendarStore.fetchEvents(client, dateRange.start, dateRange.end);
           }
+          // Refresh tasks when calendar events change (e.g. task created via CalDAV)
+          const { useTaskStore } = await import('./task-store');
+          const taskStore = useTaskStore.getState();
+          if (taskStore.tasks.length > 0 || calendarStore.viewMode === 'tasks') {
+            taskStore.fetchTasks(client);
+          }
         }
       }