Paint the bar around the Dynamic Island in the theme's background (#147)

The page named no theme-color, so the status bar iOS draws around the Dynamic Island was its own
black or white, a band across the top of every theme. The page and the admin page now carry one,
set to the page's background whenever the theme or mode changes; Android's toolbar takes it too.
Not seen on a phone yet: the browser test checks the colour matches, not what iOS does with it.

A browser test at 360px covers this and #146's row of buttons. The stub DOM gains
getComputedStyle, which every browser has.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/dom-stub.js

@@ -66,6 +66,7 @@ function makeContext(html, script, { media = false } = {}) {
     EventSource: function () { this.close = () => {}; },
     MediaMetadata: function () {},
     Blob: function () {},
+    getComputedStyle: () => ({ backgroundColor: '' }),
     setTimeout, clearTimeout, setInterval, clearInterval,
     confirm: () => false, prompt: () => null, alert(){},
     Date, Math, JSON, Object, Array, String, Number, Promise, Error, FormData: function(){},