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/gitea/client.go

@@ -706,6 +706,45 @@ func (c *Client) SubmitReview(ctx context.Context, token, owner, repo string, in
 	return nil
 }
 
+// RenderMarkdown renders raw markdown text to HTML using the Gitea API.
+// Falls back to the raw text if the API call fails.
+func (c *Client) RenderMarkdown(ctx context.Context, token, text string) (string, error) {
+	payload, err := json.Marshal(map[string]string{
+		"Text": text,
+		"Mode": "gfm",
+	})
+	if err != nil {
+		return text, fmt.Errorf("marshaling markdown request: %w", err)
+	}
+
+	url := c.baseURL + "/api/v1/markdown"
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, url, strings.NewReader(string(payload)))
+	if err != nil {
+		return text, fmt.Errorf("creating markdown request: %w", err)
+	}
+
+	req.Header.Set("Authorization", "token "+token)
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Accept", "text/html")
+
+	resp, err := c.httpClient.Do(req)
+	if err != nil {
+		return text, fmt.Errorf("executing markdown request: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode >= 400 {
+		return text, fmt.Errorf("markdown API error %d", resp.StatusCode)
+	}
+
+	rendered, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return text, fmt.Errorf("reading markdown response: %w", err)
+	}
+
+	return string(rendered), nil
+}
+
 // priorityScore returns a numeric score for sorting (lower = higher priority).
 func priorityScore(labels []string) int {
 	for _, l := range labels {