Add the 'eyes' banner design; seed banners only on select sub pages

Banner designs can now ship arbitrary markup as banner.html (canvas +
style + script), taking precedence over banner.svg; the artwork is
inlined in a div[data-design] wrapper either way, which the editor's
live preview preserves (and never re-runs its scripts).

The bundled 'eyes' design (themes/eyes/banner.html + banner.css sizing
the stage) replaces the eyes canvas script that was embedded in the
notes-on-urls page's own banner — the page now just picks the design.
Seeds set banners only on select sub pages (the-long-read gradient,
canvas-nights stars): the front page no longer gets a banner image and
shows the theme's default design. Seed entries carry a banner_design
field.

frontend/src/assets/pagerite.css

@@ -160,6 +160,15 @@ body {
   will-change: transform;
 }
 
+/* The design artwork wrapper (div[data-design]) takes the sizing and
+   parallax above; an svg inside it fills it (it is what used to be the
+   direct child before designs got a wrapper). */
+#page-banner [data-design]>svg {
+  display: block;
+  width: 100%;
+  height: 100%;
+}
+
 #brand,
 #nav {
   position: relative;