feat(engine): theme-store preview thumbnails (Phase H part 2)

The live catalog has advertised preview_url per theme since the store
shipped, but the client never fetched it — rows were text-only. The
modal now downloads each advertised preview PNG off the main thread
(same AsyncComputeTaskPool + Tokio pattern as the catalog fetch and
avatar image), decodes it into an Image asset, and renders a thumbnail
sized by the theme's own card_aspect at the head of the row. Previews
pop in as they arrive and are cached for the session, so reopening the
store is instant. Failures log and leave the row text-only; the decode
path is skipped entirely under MinimalPlugins (no Assets<Image>).

solitaire_data grows ThemeStoreClient::fetch_preview with a 512 KB cap
(previews carry no checksum — decorative only).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

solitaire_data/src/theme_store_client.rs

@@ -19,6 +19,10 @@ use thiserror::Error;
 /// so downloading it is pure waste.
 pub const MAX_THEME_DOWNLOAD_BYTES: u64 = 20 * 1024 * 1024;
 
+/// Hard cap on a preview PNG download — previews are small decorative
+/// thumbnails; anything past this is a misconfigured server.
+pub const MAX_PREVIEW_BYTES: u64 = 512 * 1024;
+
 /// Errors surfaced by [`ThemeStoreClient`].
 #[derive(Debug, Error)]
 pub enum ThemeStoreError {
@@ -110,6 +114,42 @@ impl ThemeStoreClient {
         verify_archive(&bytes, entry)?;
         Ok(bytes.to_vec())
     }
+
+    /// Fetch the preview PNG the catalog advertises for `entry`.
+    ///
+    /// Returns `Http(404)` when the entry carries no `preview_url` —
+    /// the same shape the server answers with when the file is absent,
+    /// so callers only handle one "no preview" case. Previews are
+    /// decorative; unlike archives they carry no checksum, only the
+    /// [`MAX_PREVIEW_BYTES`] size cap.
+    pub async fn fetch_preview(
+        &self,
+        entry: &ThemeCatalogEntry,
+    ) -> Result<Vec<u8>, ThemeStoreError> {
+        let Some(path) = entry.preview_url.as_deref() else {
+            return Err(ThemeStoreError::Http(404));
+        };
+        let resp = self
+            .client
+            .get(format!("{}{}", self.base_url, path))
+            .send()
+            .await
+            .map_err(|e| ThemeStoreError::Network(e.to_string()))?;
+        if !resp.status().is_success() {
+            return Err(ThemeStoreError::Http(resp.status().as_u16()));
+        }
+        let bytes = resp
+            .bytes()
+            .await
+            .map_err(|e| ThemeStoreError::Network(e.to_string()))?;
+        if bytes.len() as u64 > MAX_PREVIEW_BYTES {
+            return Err(ThemeStoreError::Oversized {
+                expected: MAX_PREVIEW_BYTES,
+                got: bytes.len() as u64,
+            });
+        }
+        Ok(bytes.to_vec())
+    }
 }
 
 /// Checks downloaded `bytes` against the catalog `entry`'s declared