Each target reads the theme from ?theme=dark|light on load and applies
it to data-theme before first paint. A MutationObserver syncs theme
changes back to the URL via replaceState, and a click handler appends
?theme= to cross-port navigation links automatically.
The outer iframe shell (dev/index.html) uses postMessage to track theme
changes from iframes and passes the param when switching tabs.
For hiccup, the server also reads ?theme= and sets data-theme on the
<html> element server-side to prevent any flash of wrong theme.
