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/SiteEditor.vue

@@ -775,12 +775,14 @@ function previewBanner() {
   if (!el) return
   if (banner.value.trim()) {
     // Own banner code supplements the design: the inlined design artwork
-    // (marked svg[data-design]) stays in place, the author code goes after
-    // it so its styles win.
-    const artwork = [...el.querySelectorAll('svg[data-design]')]
+    // (marked with [data-design]) is detached while the author code is
+    // swapped in (so runScripts never re-runs the design's own scripts),
+    // then put back first — author code stays last so its styles win.
+    const artwork = [...el.querySelectorAll('[data-design]')]
+    for (const a of artwork) a.remove()
     el.innerHTML = banner.value
-    el.prepend(...artwork)
     runScripts(el)
+    el.prepend(...artwork)
   } else {
     // No banner code of its own: the region must show the inherited/design
     // banner — re-render from the server (an empty write here would wipe it).