feat: add thumbnail previews to admin panel for albums and hero images

- Add /api/admin/thumbnail/[id] proxy endpoint for serving Immich thumbnails
- Show album cover thumbnails in AlbumCard components (subpages + standalone)
- Show actual image thumbnails in AlbumPicker modal instead of emoji placeholders
- Show small thumbnail previews for hero image entries
- Add corresponding CSS styles for all thumbnail elements

app/admin/admin.css

@@ -425,6 +425,14 @@
   border-radius: 6px;
 }
 
+.hero-item-thumb {
+  width: 32px;
+  height: 32px;
+  object-fit: cover;
+  border-radius: 4px;
+  background: #222;
+}
+
 .hero-uuid {
   font-family: monospace;
   font-size: 0.75rem;
@@ -455,6 +463,22 @@
   align-items: center;
   justify-content: space-between;
   padding: 0.6rem 0.75rem;
+  gap: 0.75rem;
+}
+
+.album-card-thumb {
+  flex-shrink: 0;
+  width: 48px;
+  height: 48px;
+  border-radius: 6px;
+  overflow: hidden;
+  background: #222;
+}
+
+.album-card-thumb img {
+  width: 100%;
+  height: 100%;
+  object-fit: cover;
 }
 
 .album-card-info {
@@ -735,6 +759,14 @@
   font-size: 1.1rem;
 }
 
+.picker-thumb-img {
+  width: 40px;
+  height: 40px;
+  object-fit: cover;
+  border-radius: 6px;
+  background: #222;
+}
+
 .picker-item-info {
   flex: 1;
   min-width: 0;