debug builds: separate applicationId so they coexist with release

Adds applicationIdSuffix=.debug and versionNameSuffix=-debug so a
side-loaded debug build can be installed alongside the Play/release
build without conflicting on package id.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/build.gradle.kts

@@ -27,6 +27,10 @@ android {
     }
 
     buildTypes {
+        debug {
+            applicationIdSuffix = ".debug"
+            versionNameSuffix = "-debug"
+        }
         release {
             signingConfig = signingConfigs.getByName("release")
             isMinifyEnabled = false