feat(ui): enforce 2:3 aspect ratio and center chatroom background

- item_character_gallery.xml: set iv_image to 2:3
- activity_chat_room.xml: apply H,2:3 ratio and center frame by constraining top/bottom to parent
- item_chat_background_image.xml: set picker item to 2:3
- align dim view constraints to match background area

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

@@ -13,10 +13,11 @@
         android:layout_height="0dp"
         android:contentDescription="@string/a11y_profile_background"
         android:scaleType="centerCrop"
-        app:layout_constraintBottom_toBottomOf="parent"
+        app:layout_constraintDimensionRatio="H,2:3"
         app:layout_constraintEnd_toEndOf="parent"
         app:layout_constraintStart_toStartOf="parent"
-        app:layout_constraintTop_toTopOf="parent" />
+        app:layout_constraintTop_toTopOf="parent"
+        app:layout_constraintBottom_toBottomOf="parent" />
 
     <!-- 딤 처리 (activity_character_detail.xml의 view_character_dim 스타일 참조) -->
     <View
@@ -24,10 +25,10 @@
         android:layout_width="0dp"
         android:layout_height="0dp"
         android:background="@color/color_99000000"
-        app:layout_constraintBottom_toBottomOf="parent"
+        app:layout_constraintBottom_toBottomOf="@id/iv_background_profile"
         app:layout_constraintEnd_toEndOf="parent"
         app:layout_constraintStart_toStartOf="parent"
-        app:layout_constraintTop_toTopOf="parent" />
+        app:layout_constraintTop_toTopOf="@id/iv_background_profile" />
 
     <!-- 헤더 영역 -->
     <androidx.constraintlayout.widget.ConstraintLayout