localize user-facing text into string resources

Move every exposed UI string into res/values/strings.xml (+ plurals) and
read it via stringResource/pluralStringResource or getString, with 27
locale files carried over from the Flutter app's Weblate translations.
Only strings whose English is unchanged reuse an old translation; reworded
ones get fresh keys.

Persisted download failures become a DownloadError enum mapped to string
resources (stored by name, unknown legacy values degrade to Failed), the
per-app locale config is generated by AGP, and StringResourcesTest guards
locale keys and placeholders.

app/build.gradle.kts

@@ -31,6 +31,10 @@ android {
         compose = true
     }
 
+    androidResources {
+        generateLocaleConfig = true
+    }
+
     compileOptions {
         sourceCompatibility = JavaVersion.VERSION_17
         targetCompatibility = JavaVersion.VERSION_17
@@ -39,6 +43,7 @@ android {
 
     lint {
         checkReleaseBuilds = false
+        disable += setOf("MissingTranslation", "MissingQuantity")
     }
 
     testOptions {