Pre-bake hero background blur/saturation, drop CSS filter

Per Copilot's performance suggestion, replace the runtime CSS
`filter: blur(1px) saturate(1.3)` on the full-viewport .hero::before
layer with a pre-processed JPEG (blurred/saturated at build time via
Pillow), avoiding the GPU compositing cost of filtering a large layer
on every paint. Visual result is unchanged.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: aclark4life

assets/css/pillow.css

@@ -5,10 +5,12 @@
    - Coral/terracotta and magenta are pulled directly from the flowers in
      the Pillow logo photo.
    - Sky blue is pulled from the desert/sky photo in the same logo.
-   - All three also appear in assets/images/psychedelic-glow.png, the
+   - All three also appear in assets/images/psychedelic-glow.jpg, the
      psychedelic rainbow texture from the original PIL-era site (art by
      Jeremy Kun), which we reuse as a subtle glow behind the hero logo for
-     a bit of the site's history and visual uniqueness.
+     a bit of the site's history and visual uniqueness. Pre-blurred and
+     pre-saturated at build time (rather than via CSS `filter`) to avoid
+     the runtime GPU cost of filtering a full-viewport layer.
    - Ink (near-black) and warm paper (off-white) anchor everything so the
      brighter accent colors stay legible and don't overwhelm the page.
    ========================================================================== */
@@ -239,14 +241,15 @@ img {
 }
 
 .hero::before {
-    /* Psychedelic art by Jeremy Kun, from the original PIL-era site */
+    /* Psychedelic art by Jeremy Kun, from the original PIL-era site.
+       Blur/saturation are pre-baked into the image itself (see the
+       palette rationale note above), so no CSS filter is needed here. */
     content: "";
     position: absolute;
     inset: 0;
-    background-image: url(../images/psychedelic-glow.png);
+    background-image: url(../images/psychedelic-glow.jpg);
     background-size: cover;
     background-position: center;
-    filter: blur(1px) saturate(1.3);
     opacity: .45;
     mix-blend-mode: screen;
     z-index: -2;