fix(tui): downgrade marked to ^15 to satisfy marked-terminal peer dep

marked-terminal@7.3.0 (latest) requires marked >=1 <16. The project had
marked@^18 which caused an ERESOLVE during npm install. Downgrade to ^15
(latest: 15.0.12) which satisfies the peer constraint.

The TUI only uses marked.use() and basic parsing — no v18-specific API
is in use, so this is a non-breaking change.

Author: delexw

package.json

@@ -27,7 +27,7 @@
     "@testing-library/dom": "^10.4.1",
     "@thesvg/react": "^2.1.3",
     "@types/marked-terminal": "^6.1.1",
-    "marked": "^18.0.0",
+    "marked": "^15.0.0",
     "marked-terminal": "^7.3.0",
     "react": "^19.2.5",
     "react-dom": "^19.2.5",