feat: Improve HTML export with markdown rendering and theme support

- Render markdown server-side using marked (tables, headings, code blocks, etc.)
- Honor user's chosen theme (light/dark) with appropriate color defaults
- Add image rendering for user messages
- Style code blocks with TUI-like ```language markers

packages/coding-agent/src/core/agent-session.ts

@@ -1483,7 +1483,8 @@ export class AgentSession {
 	 * @returns Path to exported file
 	 */
 	exportToHtml(outputPath?: string): string {
-		return exportSessionToHtml(this.sessionManager, this.state, outputPath);
+		const themeName = this.settingsManager.getTheme();
+		return exportSessionToHtml(this.sessionManager, this.state, { outputPath, themeName });
 	}
 
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