1.5: shrink and optimize release builds with R8

Release APK goes from 115 KB to 47 KB. Shizuku.newProcess is kept explicitly
because ShizukuGrant reaches it by reflection.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app/build.gradle

@@ -10,13 +10,15 @@ android {
         applicationId "com.lpw.graypause"
         minSdk 33
         targetSdk 36
-        versionCode 5
-        versionName "1.4"
+        versionCode 6
+        versionName "1.5"
     }
 
     buildTypes {
         release {
-            minifyEnabled false
+            minifyEnabled true
+            shrinkResources true
+            proguardFiles getDefaultProguardFile("proguard-android-optimize.txt"), "proguard-rules.pro"
         }
     }