Add a test for the 500-char changelog limit

Both F-Droid and Google Play cap release notes at 500 characters. Check every
metadata changelog in the unit test suite (which CI already runs), and trim the
two old Flutter changelogs that were over the limit.

app/src/test/kotlin/com/subtracks/metadata/ChangelogTest.kt

@@ -0,0 +1,33 @@
+package com.subtracks.metadata
+
+import org.junit.Assert.assertTrue
+import org.junit.Test
+import java.io.File
+
+class ChangelogTest {
+    @Test
+    fun changelogsFitStoreLimit() {
+        val root =
+            generateSequence(File(".").canonicalFile) { it.parentFile }
+                .first { File(it, "settings.gradle.kts").isFile }
+        val files =
+            File(root, "metadata")
+                .listFiles()
+                .orEmpty()
+                .flatMap { locale -> File(locale, "changelogs").listFiles().orEmpty().toList() }
+                .filter { it.isFile && it.extension == "txt" }
+        assertTrue("no changelog files found under metadata/", files.isNotEmpty())
+        files.forEach { file ->
+            val text = file.readText()
+            val length = text.codePointCount(0, text.length)
+            assertTrue(
+                "metadata changelogs must be at most $LIMIT characters; ${file.name} is $length",
+                length <= LIMIT,
+            )
+        }
+    }
+
+    private companion object {
+        const val LIMIT = 500
+    }
+}