feat: Implement comprehensive validation system for DailyNotification

- Add URL requirement validation
- Add time format validation (24-hour format)
- Add timezone validation using Intl.DateTimeFormat
- Add retry count and interval range validation
- Add content handler validation with timeout support
- Add basic schedule conflict detection
- Update validation to run before plugin calls
- Make validation methods async for content handler testing
- All validation now happens in DailyNotification class before calling native plugin

tests/edge-cases.test.ts

@@ -174,7 +174,7 @@ describe('DailyNotification Edge Cases', () => {
             }
           },
         })
-      ).rejects.toThrow('Invalid response format');
+      ).rejects.toThrow('Content handler validation failed');
     });
   });
 
@@ -200,9 +200,8 @@ describe('DailyNotification Edge Cases', () => {
     it('should handle invalid content handler responses', async () => {
       const invalidHandler = async () => {
         return {
-          title: 'Invalid Content',
+          title: '', // Empty title should fail validation
           body: 'Missing required data',
-          // Missing required fields
           data: { timestamp: new Date().toISOString() },
         };
       };