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/geocities.css

@@ -315,6 +315,20 @@
 [data-style="geocities"] .playlist-card-delete-btn,
 [data-style="geocities"] .emoji-picker-float [frimousse-emoji],
 [data-style="geocities"] .emoji-picker-close-btn,
+/* Turn the tint up and give it hard diagonal bands — a flat wash reads as an
+   unstyled gap in a theme built on saturation. */
+[data-style="geocities"] {
+  --thumb-tint-strength: 55%;
+}
+
+[data-style="geocities"] .thumb-placeholder {
+  background-image: repeating-linear-gradient(
+    135deg,
+    rgba(255, 255, 255, 0.14) 0 8px,
+    rgba(0, 0, 0, 0.14) 8px 16px
+  );
+}
+
 [data-style="geocities"] .btn--ghost,
 [data-style="geocities"] .new-item-toggle,
 [data-style="geocities"] .audio-player-btn,