Show validation errors in domain creation form

Frontend now displays backend error messages when domain creation fails.

frontend/src/routes/domains/+page.svelte

@@ -18,6 +18,7 @@
   let loading = $state(true);
   let showModal = $state(false);
   let newDomain = $state({ name: '' });
+  let createError = $state('');
 
   async function fetchWithAuth(url: string) {
     const token = localStorage.getItem('token');
@@ -48,18 +49,23 @@
   }
 
   async function createDomain() {
+    createError = '';
     try {
       const res = await fetch('/api/domains', {
         method: 'POST',
         headers: { 'Content-Type': 'application/json', Authorization: `Bearer ${localStorage.getItem('token')}` },
         body: JSON.stringify(newDomain)
       });
+      const data = await res.json();
       if (res.ok) {
         showModal = false;
         newDomain = { name: '' };
         await loadData();
+      } else {
+        createError = data.error || 'Failed to create domain';
       }
     } catch (e) {
+      createError = 'Failed to create domain';
       console.error('Failed to create domain:', e);
     }
   }
@@ -134,7 +140,7 @@
 {/if}
 
 {#if showModal}
-  <button class="modal-backdrop" onclick={() => showModal = false} aria-label="Close modal"></button>
+  <button class="modal-backdrop" onclick={() => { showModal = false; createError = ''; }} aria-label="Close modal"></button>
   <div class="modal">
     <h3>Add Domain</h3>
     <form onsubmit={(e) => { e.preventDefault(); createDomain(); }}>
@@ -142,8 +148,11 @@
         Domain name:
         <input type="text" bind:value={newDomain.name} placeholder="example.org" required />
       </label>
+      {#if createError}
+        <p class="error">{createError}</p>
+      {/if}
       <div class="actions">
-        <button type="button" onclick={() => showModal = false}>Cancel</button>
+        <button type="button" onclick={() => { showModal = false; createError = ''; }}>Cancel</button>
         <button type="submit">Create</button>
       </div>
     </form>
@@ -274,6 +283,14 @@
     border-radius: 4px;
   }
 
+  .modal .error {
+    color: #e74c3c;
+    background: #fdecea;
+    padding: 0.5rem;
+    border-radius: 4px;
+    margin: 0;
+  }
+
   .actions {
     display: flex;
     gap: 1rem;