feat: add label multi-select to Create Issue form

Add dynamic label selection when creating a new issue:
- New GET /issues/new/labels endpoint returns label checkboxes for a repo
- When a repo is selected, labels are fetched via HTMX and displayed as
  checkboxes with colored label badges
- CreateIssue handler now parses label_ids from form and passes them to
  the Gitea API
- Shows "No labels available" message when repo has no labels

Closes leeworks-agents/gitea-mobile#67

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

internal/templates/create_issue.html

@@ -20,6 +20,11 @@
     <input type="hidden" name="repo" id="repo-input">
   </div>
 
+  <div class="form-group" id="label-section" style="display:none;">
+    <label>Labels</label>
+    <div id="label-list"></div>
+  </div>
+
   <div class="form-group">
     <label for="title">Title</label>
     <input type="text" id="title" name="title" placeholder="Issue title" required>
@@ -52,7 +57,19 @@
       }
     }
 
-    repoSelect.addEventListener('change', splitOwnerRepo);
+    repoSelect.addEventListener('change', function() {
+      splitOwnerRepo();
+      var labelSection = document.getElementById('label-section');
+      var labelList = document.getElementById('label-list');
+      if (ownerInput.value && repoInput.value) {
+        labelList.innerHTML = '<span class="empty">Loading labels...</span>';
+        labelSection.style.display = 'block';
+        htmx.ajax('GET', '/issues/new/labels?owner=' + encodeURIComponent(ownerInput.value) + '&repo=' + encodeURIComponent(repoInput.value), {target: '#label-list', swap: 'innerHTML'});
+      } else {
+        labelSection.style.display = 'none';
+        labelList.innerHTML = '';
+      }
+    });
 
     // Validate before HTMX submit.
     document.getElementById('create-issue-form').addEventListener('htmx:configRequest', function(evt) {