perf: stop per-cover HttpClient churn (Android) + scroll-arrow reflow (web)

Android: MusicRepository.coverArtUrl/streamUrl built a fresh SubsonicClient
per call — each spins up (and never closes) a Ktor HttpClient. That ran for
every cover on every recomposition, causing heavy GC/alloc churn (and a leak)
that showed up as scroll/tab lag. Reuse the cached ClientHolder client for URL
building instead.

Web: the home scroll-row arrows called getBoundingClientRect on every scroll
event (layout thrashing). Split it: only toggle arrow visibility on scroll
(rAF-throttled); do the getBoundingClientRect vertical-centering only on
resize/content change.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

android/app/src/main/kotlin/com/novasonic/app/data/MusicRepository.kt

@@ -76,8 +76,9 @@ class MusicRepository(private val context: Context) {
             }
         }
         val creds = lastCredentials ?: return null
-        return SubsonicClient(creds.serverUrl, creds.username, creds.password)
-            .coverArtUrl(id ?: return null, size)
+        // Reuse the cached client — building a fresh SubsonicClient here spins up a whole Ktor
+        // HttpClient per cover (and never closes it), which is heavy GC/alloc churn while scrolling.
+        return ClientHolder.get(creds).coverArtUrl(id ?: return null, size)
     }
 
     fun streamUrl(id: String): String? {
@@ -86,7 +87,7 @@ class MusicRepository(private val context: Context) {
             if (f.exists()) return Uri.fromFile(f).toString()
         }
         val creds = lastCredentials ?: return null
-        return SubsonicClient(creds.serverUrl, creds.username, creds.password).streamUrl(id)
+        return ClientHolder.get(creds).streamUrl(id)   // reuse cached client (no per-call HttpClient)
     }
 
     private fun offlineRepo(): OfflineRepository? = runCatching {