mqtt setup panel: clipboard fallback for http lan, fill webhook url

two issues surfaced on unraid over plain http:

copy buttons: navigator.clipboard is undefined in non-secure contexts.
fall back to a hidden textarea + document.execCommand('copy'), and
surface a 'select & ⌘C' hint if even that fails.

webhook url: the field is only used by http destinations but the plugin
form requires a value regardless. put the broker url there (mqtt://host:
port) so validation passes and it's still obvious what it points at.

package.json

@@ -1,6 +1,6 @@
 {
 	"name": "netfelix-audio-fix",
-	"version": "2026.04.14.3",
+	"version": "2026.04.14.4",
 	"scripts": {
 		"dev:server": "NODE_ENV=development bun --hot server/index.tsx",
 		"dev:client": "vite",