Fix copy buttons on non-secure (plain http) deployments

`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.

src/hooks/use-clipboard-fallback.ts

@@ -0,0 +1,13 @@
+import { useEffect } from "react";
+import { installClipboardFallback } from "../utils/copy-text-to-clipboard";
+
+/**
+ * Installs the insecure-context clipboard fallback once the app mounts, so
+ * twui components calling `navigator.clipboard` directly also work over plain
+ * http on a LAN IP.
+ */
+export default function useClipboardFallback() {
+    useEffect(() => {
+        installClipboardFallback();
+    }, []);
+}