`navigator.clipboard` only exists in secure contexts, so every copy button
threw on plain-http LAN/IP deployments and falsely reported failure.
Add copyTextToClipboard(), which uses the Clipboard API on secure origins
and falls back to a hidden textarea + `execCommand("copy")` otherwise
(restoring focus/selection and always cleaning the element up). CopyButton
now reports success only when the text actually reached the clipboard.
installClipboardFallback() polyfills a working `navigator.clipboard` on
insecure pages so twui's CodeBlock/CopySlug/Input copy icons work too,
without editing twui sources. It is installed once from AppProviders.
