batch3 T6: usePanes hook + Shiki integration in CodeBlock

- hooks/usePanes: per-session panes CRUD; debounced (300ms) state PATCH;
  immediate position-change PATCH with refresh
- CodeBlock: shiki async highlighting via codeToHtml + github-dark theme;
  LANG_MAP for ts/tsx/js/jsx/py/go/rs/rb/java/c/cpp/cs/php/sh/yaml/json/
  toml/md/sql/dockerfile/html/css; falls back to plain pre on unknown lang
  or async failure
- package.json: + shiki

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/web/package.json

@@ -23,6 +23,7 @@
     "react-router-dom": "^6.26.0",
     "remark-gfm": "^4.0.1",
     "shadcn": "^4.7.0",
+    "shiki": "^1.29.2",
     "sonner": "^2.0.7",
     "tailwind-merge": "^3.6.0",
     "tw-animate-css": "^1.4.0"