Implement Touch-to-Focus (28_FOCUS_POINT.md): cache MediaPipe category mask, add focus ring view, class-based alpha selection

android-project/app/src/main/res/drawable/bg_focus_ring.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="utf-8"?>
+<shape xmlns:android="http://schemas.android.com/apk/res/android"
+    android:shape="oval">
+    <stroke
+        android:width="2dp"
+        android:color="#FFCC00" />
+    <solid android:color="#00000000" />
+</shape>

android-project/app/src/main/res/layout/activity_main.xml

@@ -143,6 +143,14 @@
             android:visibility="gone"
             android:text="3" />
 
+        <!-- Vòng tròn định vị lấy nét (Focus Ring) khi chạm vào Viewfinder -->
+        <View
+            android:id="@+id/viewFocusRing"
+            android:layout_width="60dp"
+            android:layout_height="60dp"
+            android:background="@drawable/bg_focus_ring"
+            android:visibility="gone" />
+
     </FrameLayout>
 
     <!-- ================= KHỐI 1: TIMELINE FRAMES & PRESETS ================= -->