feat: add comments thread to PR detail view

Fetch and display PR comments in the pull request detail page,
using the same Gitea issue comments API endpoint. Shows author,
timestamp, and body for each comment, with a friendly empty state
when no comments exist.

Closes leeworks-agents/gitea-mobile#81

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

internal/templates/pull_detail.html

@@ -46,4 +46,21 @@
     <button type="submit" class="btn btn-primary">Submit Review</button>
   </form>
 </div>
+
+{{if .Comments}}
+<h2>Comments</h2>
+<div id="comments-list">
+{{range .Comments}}
+<div class="comment">
+  <div class="comment-header">
+    <strong>{{.User}}</strong>
+    <span>{{.CreatedAt}}</span>
+  </div>
+  <div class="comment-body">{{.Body}}</div>
+</div>
+{{end}}
+</div>
+{{else}}
+<p class="empty" style="margin-top:1rem;">No comments yet.</p>
+{{end}}
 {{end}}