fix: validate owner/repo split in create_issue.html before submission

Add client-side validation to ensure a repository is selected before
form submission. Split owner/repo on both change and submit events.
Show inline error messages via form-error div. Update CreateIssue
handler to return HTMX-friendly HTML error fragments on 400/500.

Closes leeworks-agents/gitea-mobile#30

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/templates/create_issue.html

@@ -1,7 +1,9 @@
 {{define "content"}}
 <h1>Create Issue</h1>
 
-<form hx-post="/issues" hx-swap="innerHTML" hx-target="#main-content">
+<div id="form-error" class="empty" style="display:none;"></div>
+
+<form id="create-issue-form" hx-post="/issues" hx-swap="innerHTML" hx-target="#main-content">
   <div class="form-group">
     <label for="repo-select">Repository</label>
     <select id="repo-select" name="owner_repo" required>
@@ -32,11 +34,43 @@
 </form>
 
 <script>
-  // Split owner/repo from select into hidden fields.
-  document.getElementById('repo-select').addEventListener('change', function() {
-    var parts = this.value.split('/');
-    document.getElementById('owner-input').value = parts[0] || '';
-    document.getElementById('repo-input').value = parts[1] || '';
-  });
+  (function() {
+    var repoSelect = document.getElementById('repo-select');
+    var ownerInput = document.getElementById('owner-input');
+    var repoInput = document.getElementById('repo-input');
+    var formError = document.getElementById('form-error');
+
+    function splitOwnerRepo() {
+      var val = repoSelect.value;
+      if (val) {
+        var parts = val.split('/');
+        ownerInput.value = parts[0] || '';
+        repoInput.value = parts[1] || '';
+      } else {
+        ownerInput.value = '';
+        repoInput.value = '';
+      }
+    }
+
+    repoSelect.addEventListener('change', splitOwnerRepo);
+
+    // Validate before HTMX submit.
+    document.getElementById('create-issue-form').addEventListener('htmx:configRequest', function(evt) {
+      splitOwnerRepo();
+      if (!ownerInput.value || !repoInput.value) {
+        evt.preventDefault();
+        formError.textContent = 'Please select a repository before submitting.';
+        formError.style.display = 'block';
+        return false;
+      }
+      formError.style.display = 'none';
+    });
+
+    // Show server-side errors inline on HTMX error responses.
+    document.getElementById('create-issue-form').addEventListener('htmx:responseError', function(evt) {
+      formError.textContent = evt.detail.xhr.responseText || 'An error occurred. Please try again.';
+      formError.style.display = 'block';
+    });
+  })();
 </script>
 {{end}}