fix(homepage): force the canvas clear of the cached wallpaper div

Removing the `background:` block from settings.yaml was not sufficient.
Homepage server-renders the wallpaper as an INLINE style on
`<div id="background">` and Next.js caches the rendered page, so the aurora
survived both the config removal and a container restart. Only a full
recreate clears that cache, and recreating this container costs an hour of
missing tab bar and i18n before it heals itself.

Adding `#background` to the canvas reset is deterministic and immediate,
and it also keeps the canvas correct if the setting is ever re-added by
accident. The existing selector missed it: the DOM is
body > div#__next > div#background, so `body > div` matched the Next.js
root, not the wallpaper layer.

Verified live rather than locally: the served page now reports no
background image, with all three canonical faces loaded and the group
eyebrows rendering as JetBrains Mono in Australis cyan.

stacks/homepage/conf/custom.css

@@ -130,11 +130,22 @@
 html,
 body,
 #page_wrapper,
+#background,
 body > div {
   background-color: var(--bg-0) !important;
   background-image: none !important;
 }
 
+/* `#background` above is load-bearing, not defensive tidiness. Homepage
+   server-renders the wallpaper as an INLINE style on `<div id="background">`,
+   and Next.js caches that rendered page — so removing the `background:` block
+   from settings.yaml is not enough on its own. A restart does not clear it;
+   only a full recreate does, and recreating this container has its own cost
+   (the tab bar and i18n go missing for up to an hour and then heal on their
+   own). Enforcing it in the stylesheet is deterministic, immediate, and also
+   means the canvas stays correct if the setting is ever re-added by accident.
+   The deliberate absence of the config block is documented in settings.yaml. */
+
 body {
   font-family: var(--font-sans) !important;
   color: var(--fg-1);

stacks/homepage/theme/australis.css.in

@@ -110,11 +110,22 @@
 html,
 body,
 #page_wrapper,
+#background,
 body > div {
   background-color: var(--bg-0) !important;
   background-image: none !important;
 }
 
+/* `#background` above is load-bearing, not defensive tidiness. Homepage
+   server-renders the wallpaper as an INLINE style on `<div id="background">`,
+   and Next.js caches that rendered page — so removing the `background:` block
+   from settings.yaml is not enough on its own. A restart does not clear it;
+   only a full recreate does, and recreating this container has its own cost
+   (the tab bar and i18n go missing for up to an hour and then heal on their
+   own). Enforcing it in the stylesheet is deterministic, immediate, and also
+   means the canvas stays correct if the setting is ever re-added by accident.
+   The deliberate absence of the config block is documented in settings.yaml. */
+
 body {
   font-family: var(--font-sans) !important;
   color: var(--fg-1);