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

package-lock.json

@@ -6539,7 +6539,8 @@
 				"diff": "^8.0.2",
 				"file-type": "^21.1.1",
 				"glob": "^11.0.3",
-				"jiti": "^2.6.1"
+				"jiti": "^2.6.1",
+				"marked": "^15.0.12"
 			},
 			"bin": {
 				"pi": "dist/cli.js"