test(android): screenshot tests for every screen

Roborazzi draws Library (phone, tablet, font scale 1.5, empty), a title,
Shares, Settings and Downloads from fixed records under Robolectric, and
build-android.sh check compares them with the committed images in
app/src/test/screenshots. The redesign changes every screen; a layout
regression now fails the gate and shows up in review as a changed image.

android/app/build.gradle.kts

@@ -6,6 +6,7 @@ plugins {
     alias(libs.plugins.kotlin.android)
     alias(libs.plugins.kotlin.compose)
     alias(libs.plugins.ktlint)
+    alias(libs.plugins.roborazzi)
 }
 
 val repositoryRoot = rootProject.projectDir.parentFile
@@ -272,6 +273,11 @@ dependencies {
     testImplementation(libs.robolectric)
     testImplementation(libs.androidx.test.core)
     testImplementation(libs.androidx.work.testing)
+    testImplementation(platform(libs.androidx.compose.bom))
+    testImplementation(libs.androidx.compose.ui.test.junit4)
+    testImplementation(libs.roborazzi)
+    testImplementation(libs.roborazzi.compose)
+    testImplementation(libs.roborazzi.junit.rule)
 
     androidTestImplementation(libs.androidx.test.junit)
     androidTestImplementation(libs.androidx.test.core)
@@ -295,6 +301,13 @@ ktlint {
     }
 }
 
+// Screenshot baselines live beside the tests that draw them, so a change to how
+// a screen looks shows up in review as a changed image. `recordRoborazziDebug`
+// rewrites them; `verifyRoborazziDebug` fails on any difference.
+roborazzi {
+    outputDir.set(file("src/test/screenshots"))
+}
+
 // The generated UniFFI directory is a main source dir, so ktlint reads it even
 // though the filter above drops every file in it. Without this, Gradle refuses
 // the build for an undeclared dependency between the two tasks.