fix(master): keep the scroll position when toggling auto mode on the overview

The mode switch was a plain form POST answered with 303 to /, so the browser
navigated to a fresh page at the top. The form now also posts via htmx and
the handler returns the services fragment for htmx requests, so the list is
swapped in place; search and machine filter stay applied. A refused auto
answers 409 with HX-Redirect to the service page, since htmx does not swap
4xx responses. Without JS the form still posts and redirects as before.

#services gets overflow-anchor:none. htmx inserts the new list before
removing the old one, and Chrome anchored scrolling on the focused switch,
so on narrow screens the page jumped to the bottom after a click, and after
any SSE refresh while a switch had focus.

Claude-Session: https://claude.ai/code/session_01GAZsp3P7Qto6JnhipxWUJQ

internal/master/web/templates/overview.html

@@ -83,7 +83,7 @@
       {{if .Self}}<span class="tag f-out">This agent</span>{{end}}
     </div>
     <div class="mode">
-      <form method="post" action="/services/{{.ID}}/mode">
+      <form method="post" action="/services/{{.ID}}/mode" hx-post="/services/{{.ID}}/mode" hx-target="#services" hx-swap="innerHTML">
         <input type="hidden" name="csrf_token" value="{{$.CSRF}}">
         <input type="hidden" name="return" value="overview">
         {{if eq .UpdateMode "auto"}}<input type="hidden" name="mode" value="manual">