feat: render issue and PR body as markdown via Gitea API

Add RenderMarkdown method to gitea client that calls POST /api/v1/markdown
to convert raw markdown text to safe HTML. Wire it into IssueDetail and
PullDetail handlers to render body content as formatted markdown.
Falls back gracefully to plain text if the API call fails.

Templates updated to use RenderedBody (template.HTML) with fallback
to raw Issue.Body/Pull.Body when rendering fails.

Closes leeworks-agents/gitea-mobile#35

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

internal/templates/issue_detail.html

@@ -9,7 +9,9 @@
     <span class="label" style="color:#{{.Color}};border:1px solid #{{.Color}}">{{.Name}}</span>
     {{end}}
   </div>
-  {{if .Issue.Body}}
+  {{if .RenderedBody}}
+  <div class="card-body markdown-body">{{.RenderedBody}}</div>
+  {{else if .Issue.Body}}
   <div class="card-body">{{.Issue.Body}}</div>
   {{end}}
 </div>