Surface WAIT_USER and add Confirm to the browser client

Ports show_user_message() from client/brewpi_gui.py: a plain <dialog>
pops with the step's user_message the instant WAIT_USER is newly
entered (tracked via a prevState comparison in onSudChanged()'s State
branch), and sends {"Sud": {"Confirm": true}} on the dialog's close
event - covers both the OK button and Escape dismissal.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0189FkmyJkY77HqsNomr1DwV

web/index.html

@@ -102,6 +102,13 @@
 	</section>
 </main>
 
+<dialog id="sud-message-dialog">
+	<p id="sud-message-text"></p>
+	<form method="dialog">
+		<button value="ok">OK</button>
+	</form>
+</dialog>
+
 <footer>
 	<div id="sud-status-line"></div>
 	<div id="env-status-line"></div>