metadata: render phone screenshots from the Navidrome demo library

Replace the placeholder-art screenshots with renders of a real library, so the
store listing shows real album and artist art and names.

The generator (DemoScreenshotTest, run with `gradle :app:demoScreenshots`) syncs
the public Navidrome demo (demo.navidrome.org) into an in-memory database using
the normal SyncService pipeline, derives the theme colours from the fetched
cover art, and renders each screen. The synced data and cover art stay in memory
and are never written to the repository; only the resulting PNGs are committed.
The test is excluded from the unit-test tasks and is not part of CI.

Also reverts the screenshot fixture palette tweak from the previous commit,
which the real art makes unnecessary.

app/build.gradle.kts

@@ -100,10 +100,12 @@ val integrationTestClasses =
         "com.subtracks.data.sync.SyncServiceIntegrationTest",
     )
 
+val demoScreenshotClass = "com.subtracks.ui.DemoScreenshotTest"
+
 tasks.withType<Test>().configureEach {
     if (name.endsWith("UnitTest")) {
         filter {
-            integrationTestClasses.forEach { excludeTestsMatching(it) }
+            (integrationTestClasses + demoScreenshotClass).forEach { excludeTestsMatching(it) }
         }
     }
     testLogging {
@@ -125,6 +127,20 @@ tasks.register<Test>("integrationTest") {
     systemProperty("prune.musicDir", providers.gradleProperty("pruneMusicDir").getOrElse(""))
 }
 
+tasks.register<Test>("demoScreenshots") {
+    description = "Records the store screenshots from a real sync of the public Navidrome demo library"
+    group = "verification"
+    val unitTest = tasks.named<Test>("testDebugUnitTest")
+    dependsOn("compileDebugUnitTestKotlin")
+    testClassesDirs = files(provider { unitTest.get().testClassesDirs })
+    classpath = files(provider { unitTest.get().classpath })
+    filter {
+        includeTestsMatching(demoScreenshotClass)
+    }
+    systemProperty("roborazzi.test.record", "true")
+    outputs.upToDateWhen { false }
+}
+
 dependencies {
     implementation(platform(libs.androidx.compose.bom))
     implementation(libs.androidx.core.ktx)