v3: generated placeholder thumbnails for pages with no preview image, and a fix for hotlink-protected ones

Thumbnails were failing in two different ways that both ended as an empty box.

Cloudflare hotlink protection answers a cross-site Referer with 403, so images
we had extracted correctly (dles.aukspot.com's og:image among them) never
rendered — every onError handler set display:none and swallowed it. The new
Thumbnail component loads with referrerPolicy="no-referrer", retries once
through /api/proxy-image for hosts that reject an empty referrer too, and only
then falls back to a placeholder.

Separately, the extraction cascade ended at the page's icon and then a guessed
/favicon.ico, so thumbnailUrl was almost never empty — just a 16x16 icon
cover-cropped into a 128x72 box. It now stops at real artwork, with faviconUrl
and accentColor (theme-color / msapplication-TileColor / mask-icon) as their own
fields. An absent thumbnailUrl finally means "no artwork", which is what makes
the placeholder possible: the site's own color mixed into the theme surface,
with its favicon centered on it, or its initial. Contrast holds for any
third-party color by construction rather than by luminance math, so nyt only
has to set --thumb-tint-strength to 0% to stay monochrome and geocities only
has to raise it. Missing accents fall back to a stable hostname-derived hue, so
rows saved before this get a tint with no backfill.

Migration 0011 reclassifies favicon-shaped thumbnailUrls on existing dumps.
The journal mosaic keeps its pull-quote and text fallbacks — the placeholder
appears there only to repair a broken image.

Also fixed: refresh silently overwrote good metadata with a failure stub, the
refresh button swallowed every error, refresh never broadcast the update,
extractBestIcon ranked SVG icons below 16x16 PNGs, and shared links with no
artwork carried no og:image at all.

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

src/pages/DumpEdit.tsx

@@ -53,6 +53,7 @@ export function DumpEdit() {
 
   const [confirmDelete, setConfirmDelete] = useState(false);
   const [refreshing, setRefreshing] = useState(false);
+  const [refreshError, setRefreshError] = useState<string | null>(null);
   const [thumbUploading, setThumbUploading] = useState(false);
 
   useEffect(() => {
@@ -85,15 +86,20 @@ export function DumpEdit() {
     if (state.status !== "loaded" || state.dump.kind !== "url") return;
 
     setRefreshing(true);
+    setRefreshError(null);
     try {
       const res = await authFetch(
         `${API_URL}/api/dumps/${state.dump.id}/refresh-metadata`,
         { method: "POST" },
       );
-      const apiResponse = await res.json();
+      const apiResponse = parseAPIResponse<RawDump>(await res.json());
       if (apiResponse.success) {
         setState({ status: "loaded", dump: deserializeDump(apiResponse.data) });
+      } else {
+        setRefreshError(apiResponse.error.message);
       }
+    } catch (err) {
+      setRefreshError(friendlyFetchError(err));
     } finally {
       setRefreshing(false);
     }
@@ -235,6 +241,9 @@ export function DumpEdit() {
                 : <Trans>Refresh metadata</Trans>}
             </button>
           )}
+          {refreshError && (
+            <p className="dump-edit-refresh-error">{refreshError}</p>
+          )}
         </div>
 
         <DumpEditForm