fix(android): resolve prefetch scheduling and permission callback issues

- Add null safety check to permission callback to prevent NPE
- Fix fetch time calculation bug that caused double subtraction
  - scheduleFetch() now accepts pre-calculated fetchTime directly
  - Calculate scheduledTime back from fetchTime for worker data
- Add structured logging (DN|FETCH_SCHEDULING) for better traceability

The permission callback was crashing with NullPointerException when
Capacitor passed a null call parameter. The prefetch scheduling had a
logic error where fetchTime was calculated twice - once in the plugin
and once in the fetcher, causing 10-minute delays instead of 5-minute.

Both issues are now fixed and verified working:
- Permission callback handles null gracefully
- Prefetch schedules correctly 5 minutes before notification
- WorkManager job fires at the correct time
- All structured logs appear in logcat

Closes prefetch scheduling investigation.

android/plugin/src/main/java/com/timesafari/dailynotification/DailyNotificationFetcher.java

@@ -72,19 +72,23 @@ public class DailyNotificationFetcher {
     /**
      * Schedule a background fetch for content
      * 
-     * @param scheduledTime When the notification is scheduled for
+     * @param fetchTime When to fetch the content (already calculated, typically 5 minutes before notification)
      */
-    public void scheduleFetch(long scheduledTime) {
+    public void scheduleFetch(long fetchTime) {
         try {
-            Log.d(TAG, "Scheduling background fetch for " + scheduledTime);
+            Log.d(TAG, "Scheduling background fetch for time: " + fetchTime);
             
-            // Calculate fetch time (5 minutes before notification)
-            long fetchTime = scheduledTime - TimeUnit.MINUTES.toMillis(5);
+            long currentTime = System.currentTimeMillis();
+            long delayMs = fetchTime - currentTime;
             
-            if (fetchTime > System.currentTimeMillis()) {
-                long delayMs = fetchTime - System.currentTimeMillis();
+            Log.d(TAG, "DN|FETCH_SCHEDULING fetch_time=" + fetchTime + 
+                      " current=" + currentTime + 
+                      " delay_ms=" + delayMs);
+            
+            if (fetchTime > currentTime) {
+                // Create work data - we need to calculate the notification time (fetchTime + 5 minutes)
+                long scheduledTime = fetchTime + TimeUnit.MINUTES.toMillis(5);
                 
-                // Create work data
                 Data inputData = new Data.Builder()
                     .putLong("scheduled_time", scheduledTime)
                     .putLong("fetch_time", fetchTime)
@@ -105,13 +109,13 @@ public class DailyNotificationFetcher {
                 Log.i(TAG, "DN|WORK_ENQUEUED work_id=" + fetchWork.getId().toString() + 
                           " fetch_at=" + fetchTime + 
                           " delay_ms=" + delayMs +
-                          " delay_hours=" + (delayMs / 3600000.0));
+                          " delay_minutes=" + (delayMs / 60000.0));
                 Log.i(TAG, "Background fetch scheduled successfully");
                 
             } else {
                 Log.w(TAG, "DN|FETCH_PAST_TIME fetch_time=" + fetchTime + 
-                          " current=" + System.currentTimeMillis() + 
-                          " past_by_ms=" + (System.currentTimeMillis() - fetchTime));
+                          " current=" + currentTime + 
+                          " past_by_ms=" + (currentTime - fetchTime));
                 scheduleImmediateFetch();
             }
             

android/plugin/src/main/java/com/timesafari/dailynotification/DailyNotificationPlugin.java

@@ -1243,6 +1243,13 @@ public class DailyNotificationPlugin extends Plugin {
     private void onPermissionResult(PluginCall call) {
         try {
             Log.d(TAG, "DEBUG: onPermissionResult callback received");
+            
+            // Guard against null call
+            if (call == null) {
+                Log.e(TAG, "Permission callback received null call - cannot process");
+                return;
+            }
+            
             Log.d(TAG, "Permission callback received");
             
             // Check if POST_NOTIFICATIONS permission was granted
@@ -1262,7 +1269,9 @@ public class DailyNotificationPlugin extends Plugin {
         } catch (Exception e) {
             Log.e(TAG, "DEBUG: Exception in onPermissionResult callback", e);
             Log.e(TAG, "Error in permission callback", e);
-            call.reject("Error processing permission result: " + e.getMessage());
+            if (call != null) {
+                call.reject("Error processing permission result: " + e.getMessage());
+            }
         }
     }