refactor(android): add shared albumCoverPath cover-URL builder

android/app/src/main/java/com/fabledsword/minstrel/models/CoverUrls.kt

@@ -0,0 +1,11 @@
+package com.fabledsword.minstrel.models
+
+/**
+ * Absolute cover URL for an album, or "" when [albumId] is blank. Uses
+ * the placeholder host that `BaseUrlInterceptor` rewrites to the live
+ * server (carrying the auth cookie); `ServerImage` passes it through,
+ * and the raw-AsyncImage cover surfaces still load it. The single
+ * album-cover URL builder for the whole client.
+ */
+fun albumCoverPath(albumId: String): String =
+    if (albumId.isEmpty()) "" else "http://placeholder.invalid/api/albums/$albumId/cover"

android/app/src/test/java/com/fabledsword/minstrel/models/CoverUrlsTest.kt

@@ -0,0 +1,16 @@
+package com.fabledsword.minstrel.models
+
+import org.junit.jupiter.api.Test
+import kotlin.test.assertEquals
+
+class CoverUrlsTest {
+    @Test
+    fun `albumCoverPath builds the placeholder cover URL`() {
+        assertEquals("http://placeholder.invalid/api/albums/a1/cover", albumCoverPath("a1"))
+    }
+
+    @Test
+    fun `albumCoverPath is empty for a blank id`() {
+        assertEquals("", albumCoverPath(""))
+    }
+}