fix(embeds): animate GIF previews; add Mixcloud/Deezer; misc embed fixes

GIF previews rendered but never played: Synapse's /thumbnail endpoint
flattens animated GIFs to a still first frame. GifCard and the generic OG
card now request the original via /download (no width/height) for GIFs, so
they animate. Guarded with shouldServeGifOriginal(): a matrix:image:size cap
(10 MB) keeps a huge self-hosted GIF on the frozen thumbnail, and the generic
card's eager <img> gains loading="lazy" (it was the one preview image missing
it) so originals stay off the wire until near the viewport.

Also adds Mixcloud + Deezer inline media embeds (iframe widgets via
parseMediaEmbed/MediaEmbedCard, matching the existing click-to-play pattern),
and fixes Deezer podcast links: they live at /show/<id>, not /podcast/<id>
(the latter 404s on Deezer's own oEmbed) — verified against the live API.

Reviewed by two agents; both findings (Deezer /show, GIF eager-load) fixed
and covered by tests. Desktop Tauri frame-src CSP updated separately.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/components/url-preview/UrlPreview.css.tsx

@@ -549,6 +549,16 @@ export const BadgeTidal = style({
   color: '#ffffff',
 });
 
+export const BadgeMixcloud = style({
+  backgroundColor: '#52aad8',
+  color: '#ffffff',
+});
+
+export const BadgeDeezer = style({
+  backgroundColor: '#a238ff',
+  color: '#ffffff',
+});
+
 // ---------------------------------------------------------------------------
 // Twitch LIVE badge
 // ---------------------------------------------------------------------------