fix: host modal — single POST endpoint for create+edit, remove hx-put conflict

- hx-put="" (empty) was causing HTMX to send wrong requests
- Now use single hx-post="/hosts" with hidden hostId field
- Server checks hostId: empty → create, present → update
- UpdateHost reads ID from both URL param and form data

app/backend/views/host_modal.html

@@ -15,11 +15,11 @@
         </button>
       </div>
 
-      <form :hx-post="editing ? null : '/hosts'"
-            :hx-put="editing ? '/hosts/' + hostId : null"
+      <form hx-post="/hosts"
             hx-target="#host-list" hx-swap="innerHTML"
             @submit="open = false; $dispatch('toast-message', { message: editing ? 'Host updated' : 'Host added' })"
             class="p-6 space-y-4 text-sm">
+        <input type="hidden" name="hostId" :value="editing ? hostId : ''">
 
         <div class="grid grid-cols-2 gap-4">
           <div class="flex flex-col gap-1.5">