feat(dev-app): register native fetcher SPI implementation

Add host app implementation of NativeNotificationContentFetcher for development app:
- Create PluginApplication extends Application to register fetcher on app startup
- Create DemoNativeFetcher implementing NativeNotificationContentFetcher interface
- Register PluginApplication in AndroidManifest.xml
- DemoNativeFetcher returns mock notification content for testing

This demonstrates the SPI pattern where host apps provide their own
content fetching implementation to the plugin for background workers.

android/app/src/main/AndroidManifest.xml

@@ -2,6 +2,7 @@
 <manifest xmlns:android="http://schemas.android.com/apk/res/android">
 
     <application
+        android:name=".PluginApplication"
         android:allowBackup="true"
         android:icon="@mipmap/ic_launcher"
         android:label="@string/app_name"

android/app/src/main/java/com/timesafari/dailynotification/DemoNativeFetcher.java

@@ -0,0 +1,85 @@
+/**
+ * DemoNativeFetcher.java
+ * 
+ * Simple demo implementation of NativeNotificationContentFetcher for the plugin demo app.
+ * Returns mock notification content for demonstration purposes.
+ * 
+ * @author Matthew Raymer
+ * @version 1.0.0
+ */
+
+package com.timesafari.dailynotification;
+
+import android.util.Log;
+import androidx.annotation.NonNull;
+import com.timesafari.dailynotification.FetchContext;
+import com.timesafari.dailynotification.NativeNotificationContentFetcher;
+import com.timesafari.dailynotification.NotificationContent;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import java.util.concurrent.CompletableFuture;
+
+/**
+ * Demo implementation of native content fetcher
+ * 
+ * Returns mock notification content for demonstration. In production host apps,
+ * this would fetch from TimeSafari API or other data sources.
+ */
+public class DemoNativeFetcher implements NativeNotificationContentFetcher {
+    
+    private static final String TAG = "DemoNativeFetcher";
+    
+    @Override
+    @NonNull
+    public CompletableFuture<List<NotificationContent>> fetchContent(
+            @NonNull FetchContext context) {
+        
+        Log.d(TAG, "DemoNativeFetcher: Fetch triggered - trigger=" + context.trigger +
+            ", scheduledTime=" + context.scheduledTime + ", fetchTime=" + context.fetchTime);
+        
+        return CompletableFuture.supplyAsync(() -> {
+            try {
+                // Simulate network delay
+                Thread.sleep(100);
+                
+                List<NotificationContent> contents = new ArrayList<>();
+                
+                // Create a demo notification
+                NotificationContent demoContent = new NotificationContent();
+                demoContent.setId("demo_notification_" + System.currentTimeMillis());
+                demoContent.setTitle("Demo Notification from Native Fetcher");
+                demoContent.setBody("This is a demo notification from the native fetcher SPI. " +
+                    "Trigger: " + context.trigger + 
+                    (context.scheduledTime != null ? 
+                        ", Scheduled: " + new java.util.Date(context.scheduledTime) : ""));
+                
+                // Use scheduled time from context, or default to 1 hour from now
+                long scheduledTimeMs = context.scheduledTime != null ?
+                    context.scheduledTime : (System.currentTimeMillis() + 3600000);
+                demoContent.setScheduledTime(scheduledTimeMs);
+                
+                // fetchTime is set automatically by NotificationContent constructor (as fetchedAt)
+                demoContent.setPriority("default");
+                demoContent.setSound(true);
+                
+                contents.add(demoContent);
+                
+                Log.i(TAG, "DemoNativeFetcher: Returning " + contents.size() + 
+                    " notification(s)");
+                
+                return contents;
+                
+            } catch (InterruptedException e) {
+                Log.e(TAG, "DemoNativeFetcher: Interrupted during fetch", e);
+                Thread.currentThread().interrupt();
+                return Collections.emptyList();
+                
+            } catch (Exception e) {
+                Log.e(TAG, "DemoNativeFetcher: Error during fetch", e);
+                return Collections.emptyList();
+            }
+        });
+    }
+}
+

android/app/src/main/java/com/timesafari/dailynotification/PluginApplication.java

@@ -0,0 +1,38 @@
+/**
+ * PluginApplication.java
+ * 
+ * Application class for the Daily Notification Plugin demo app.
+ * Registers the native content fetcher SPI implementation.
+ * 
+ * @author Matthew Raymer
+ * @version 1.0.0
+ */
+
+package com.timesafari.dailynotification;
+
+import android.app.Application;
+import android.util.Log;
+import com.timesafari.dailynotification.DailyNotificationPlugin;
+import com.timesafari.dailynotification.NativeNotificationContentFetcher;
+
+/**
+ * Application class that registers native fetcher for plugin demo app
+ */
+public class PluginApplication extends Application {
+    
+    private static final String TAG = "PluginApplication";
+    
+    @Override
+    public void onCreate() {
+        super.onCreate();
+        
+        Log.i(TAG, "Initializing Daily Notification Plugin demo app");
+        
+        // Register demo native fetcher
+        NativeNotificationContentFetcher demoFetcher = new DemoNativeFetcher();
+        DailyNotificationPlugin.setNativeFetcher(demoFetcher);
+        
+        Log.i(TAG, "Demo native fetcher registered: " + demoFetcher.getClass().getName());
+    }
+}
+