feat(android): AlbumCard cover fallback for cached-only albums

The cached_albums → AlbumRef mapper drops coverUrl (only the
coverPath column is stored; the regular API's cover_url isn't
mirrored). Result: AlbumCards rendered from cache — Library Albums
tab, ArtistDetail album grid, Home Recently-added/Rediscover rows
on cold start — showed only the Lucide.Disc3 placeholder.

Same placeholder-URL trick as TrackRef.coverUrl:

  - models/AlbumRef.kt — adds `displayCoverUrl` computed property
    that returns the server-given coverUrl when populated, falls
    back to `http://placeholder.invalid/api/albums/{id}/cover`
    otherwise. BaseUrlInterceptor rewrites the host; Coil's shared
    OkHttp picks up the auth cookie. The original `coverUrl` field
    is preserved so callers that need to distinguish
    "server-provided" from "derived" can.

  - library/widgets/AlbumCard.kt — switches the branch from
    `album.coverUrl.isEmpty()` to `album.id.isEmpty()`. The cover
    Box gets a surfaceVariant background so failed image loads
    (e.g. album server-side without art) degrade to a tinted
    square rather than transparent. A proper error-slot fallback
    icon is a future refinement.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

android/app/src/main/java/com/fabledsword/minstrel/library/widgets/AlbumCard.kt

@@ -1,5 +1,6 @@
 package com.fabledsword.minstrel.library.widgets
 
+import androidx.compose.foundation.background
 import androidx.compose.foundation.clickable
 import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.Column
@@ -49,10 +50,11 @@ fun AlbumCard(
             Box(
                 modifier = Modifier
                     .size(144.dp)
-                    .clip(RoundedCornerShape(FabledSwordFlatTokens.radiusSm)),
+                    .clip(RoundedCornerShape(FabledSwordFlatTokens.radiusSm))
+                    .background(MaterialTheme.colorScheme.surfaceVariant),
                 contentAlignment = Alignment.Center,
             ) {
-                if (album.coverUrl.isEmpty()) {
+                if (album.id.isEmpty()) {
                     Icon(
                         imageVector = Lucide.Disc3,
                         contentDescription = null,
@@ -60,8 +62,15 @@ fun AlbumCard(
                         modifier = Modifier.size(56.dp),
                     )
                 } else {
+                    // Use displayCoverUrl so cached-only albums (which
+                    // drop coverUrl in the entity→domain mapper) still
+                    // hit /api/albums/{id}/cover via the BaseUrlInterceptor
+                    // placeholder rewrite. On load failure (album with
+                    // no cover server-side) the Box's surfaceVariant
+                    // background shows through — a Disc3 error-slot
+                    // fallback is a future refinement.
                     AsyncImage(
-                        model = album.coverUrl,
+                        model = album.displayCoverUrl,
                         contentDescription = album.title,
                         modifier = Modifier.fillMaxSize(),
                         contentScale = ContentScale.Crop,

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

@@ -19,4 +19,18 @@ data class AlbumRef(
     val trackCount: Int = 0,
     val durationSec: Int = 0,
     val coverUrl: String = "",
-)
+) {
+    /**
+     * Cover URL the UI should render. Returns the server-provided
+     * `coverUrl` when populated (regular API responses always carry it),
+     * otherwise derives `/api/albums/{id}/cover` via the placeholder
+     * host that `BaseUrlInterceptor` rewrites to the live server URL.
+     *
+     * The fallback path matters because the cached_albums entity →
+     * AlbumRef mapper drops coverUrl — without this property,
+     * AlbumCards rendered from cache (Library Albums tab, ArtistDetail
+     * grid, Home rows on cold-start) would show no artwork at all.
+     */
+    val displayCoverUrl: String
+        get() = coverUrl.ifEmpty { "http://placeholder.invalid/api/albums/$id/cover" }
+}