chore(deps): declare shikiji-core as direct dependency

useMarkdown.hook.ts imports types from shikiji-core, but it was only
present transitively via markdown-it-shikiji. pnpm's strict resolution
blocks importing undeclared transitives, breaking type-check.

Pinned to 0.10.2 to match the version markdown-it-shikiji@0.10.2 pulls
in. The shikiji line is deprecated upstream in favor of shiki; a real
migration is out of scope here.

package.json

@@ -61,6 +61,7 @@
     "register-service-worker": "^1.7.2",
     "retrobus": "^1.9.4",
     "sanitize-html": "^2.17.0",
+    "shikiji-core": "0.10.2",
     "vue": "^3.5.18",
     "vue-i18n": "^11.1.11",
     "vue-router": "^4.5.1",