feat: display assignee avatar in issue list rows

Replace plain-text assignee login with a circular avatar image using
the existing .avatar CSS class. Includes title attribute for
accessibility. Unassigned issues show no avatar.

Closes leeworks-agents/gitea-mobile#98

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

internal/templates/issues.html

@@ -8,7 +8,7 @@
       <span class="label" style="color:#{{.Color}};border:1px solid #{{.Color}}">{{.Name}}</span>
       {{end}}
       {{if .Assignee}}
-      <span>{{.Assignee.Login}}</span>
+      <img src="{{.Assignee.AvatarURL}}" alt="{{.Assignee.Login}}" class="avatar" title="Assigned to {{.Assignee.Login}}">
       {{end}}
     </div>
   </div>