feat(glass): dev-prototype feed backdrop (blurred rotating thumbnail)

Idea from UAT-2: give the feeds the same 'content behind glass' lift the Plex
detail page gets. New FeedBackdrop paints a faint, slowly-rotating feed thumbnail
as a fixed backdrop on <main> (the proven art-backdrop path — a fixed z:-1 layer
is occluded by the root's opaque bg-bg). Blur = tiny-canvas downscale + cover
upscale (no CSS filter; ytimg is CORS-readable so it uses a data-URL, else raw).
Flat --bg stays as the 'perf mode' look. Behind the GlassTuner toggle + a
Backdrop-fade slider (--feedbg-fade) so it's judged/tuned in UAT before anything
is committed for real.

frontend/src/index.css

@@ -21,6 +21,10 @@
   --glass-edge: 10%; /* extra edge-light mixed into the TOP border (#fff N%) — 0 = off */
   --glass-scrim: 14%; /* under-content scrim: darkens surfaces toward --bg for text contrast — 0 = off */
   --ambient: 1.1; /* multiplier on the ambient accent backdrop pools */
+
+  /* Dev-prototype feed backdrop (see <FeedBackdrop/>): how strongly the faded --bg overlay hides the
+     blurred thumbnail — higher = fainter image. Only in effect while the backdrop is toggled on. */
+  --feedbg-fade: 86%;
 }
 
 /* ===== Rich-backdrop scope: surfaces that float over real imagery (Plex art hero/detail, and