feat: merge three camera modes into unified CameraActivity

- Added mode toggle button to switch between photo, watermark, and compose modes
- Integrated watermark functionality from WatermarkCameraActivity
- Integrated simple camera functionality from SimpleCameraActivity
- Updated MainActivity to launch CameraActivity directly
- Cleaned up AndroidManifest.xml by removing unused activities
- Updated layout with mode selection button
- All three camera modes now available in single unified interface

app/src/main/res/layout/activity_main.xml

@@ -5,7 +5,7 @@
     xmlns:tools="http://schemas.android.com/tools"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
-    tools:context=".MainActivity">
+    tools:context=".CameraActivity">
 
     <!-- CameraView for displaying the camera preview -->
     <androidx.camera.view.PreviewView
@@ -35,6 +35,16 @@
             android:orientation="horizontal"
             android:gravity="center">
 
+            <!-- Mode button -->
+            <ImageButton
+                android:id="@+id/modeButton"
+                android:layout_width="48dp"
+                android:layout_height="48dp"
+                android:layout_margin="16dp"
+                android:background="?attr/selectableItemBackgroundBorderless"
+                android:src="@drawable/ic_camera"
+                android:contentDescription="Change mode" />
+
             <!-- Capture button -->
             <ImageButton
                 android:id="@+id/captureButton"