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/themes/nyt.css

@@ -587,10 +587,17 @@
 [data-style="nyt"] .rich-content-compact-icon,
 [data-style="nyt"] .journal-card-glyph,
 [data-style="nyt"] .fdz__file-icon,
+[data-style="nyt"] .thumb-placeholder-icon,
 [data-style="nyt"] .empty-state {
   filter: grayscale(1);
 }
 
+/* Monochrome by design — a dumped site's brand color doesn't get to tint the
+   page, so the placeholder is the theme's own surface with a gray glyph. */
+[data-style="nyt"] {
+  --thumb-tint-strength: 0%;
+}
+
 /* ── Tighter, continuous ruled feed (no inter-item gap; the hairline
    bottom rules read as one column). Even vertical padding per item. ── */
 [data-style="nyt"] .dump-feed {