build: give the nightly a distinct icon and drop the longer label

The 17-character 'subtracks nightly' label just truncates in the launcher,
so revert the app_name override (debug and nightly show 'subtracks' like
the release) and distinguish the nightly with a recoloured launcher icon
(amber mark) instead. The debug and nightly variants share src/nightly/res,
so installDebug and installNightly look the same.

app/build.gradle.kts

@@ -30,7 +30,6 @@ android {
     buildFeatures {
         compose = true
         buildConfig = true
-        resValues = true
     }
 
     androidResources {
@@ -99,10 +98,13 @@ android {
         }
     }
 
+    sourceSets {
+        getByName("debug").res.srcDir("src/nightly/res")
+    }
+
     buildTypes {
         debug {
             applicationIdSuffix = ".nightly"
-            resValue("string", "app_name", "subtracks nightly")
         }
         release {
             isMinifyEnabled = true
@@ -115,7 +117,6 @@ android {
             initWith(getByName("release"))
             applicationIdSuffix = ".nightly"
             signingConfig = signingConfigs.getByName("debug")
-            resValue("string", "app_name", "subtracks nightly")
         }
     }
 }