perf(code): replace react-syntax-highlighter and prismjs with the Shiki worker

Route all non-markdown code highlighting through the off-main-thread Shiki
worker, removing react-syntax-highlighter and prismjs entirely.

- Extend the worker with highlightLines: tokenize a whole block once and return
  per-line inner HTML, so per-line layouts (diffs, gutters, virtualization) make
  one worker call instead of one highlighter per line.
- Add shared WorkerHighlightedCode (whole-block) and useWorkerHighlightedLines
  (per-line) primitives. Colors resolve via the --md-syntax-* CSS variables, so
  theme changes never re-highlight.
- Migrate all 12 react-syntax-highlighter call sites: PermissionCard,
  ToolPart, ContextSidebarTab, ToolOutputDialog (whole block) and
  DiffPreview/WritePreview (per line).
- Migrate VirtualizedCodeBlock off prismjs to the worker, keeping virtua
  virtualization; whole-block tokenization also restores cross-line syntax
  context that per-line highlighting lost.
- Drop react-syntax-highlighter (+types) from ui and web, prismjs (+types) from
  ui, and the orphaned create-element type shim.

packages/ui/package.json

@@ -44,7 +44,6 @@
     "@pierre/diffs": "1.3.0-beta.4",
     "@replit/codemirror-vim": "^6.3.0",
     "@simplewebauthn/browser": "13.3.0",
-    "@types/react-syntax-highlighter": "^15.5.13",
     "@xenova/transformers": "^2.17.2",
     "@zumer/snapdom": "^2.12.0",
     "beautiful-mermaid": "^1.1.3",
@@ -64,12 +63,10 @@
     "morphdom": "^2.7.7",
     "motion": "^12.23.24",
     "next-themes": "^0.4.6",
-    "prismjs": "^1.30.0",
     "qrcode": "^1.5.4",
     "react": "^19.1.1",
     "react-dom": "^19.1.1",
     "react-drawio": "1.0.7",
-    "react-syntax-highlighter": "^15.6.6",
     "rehype-katex": "^7.0.1",
     "remark-math": "^6.0.0",
     "remend": "^1.2.1",
@@ -87,7 +84,6 @@
     "@eslint/js": "^9.33.0",
     "@tailwindcss/postcss": "^4.0.0",
     "@types/node": "^24.3.1",
-    "@types/prismjs": "^1.26.6",
     "@types/qrcode": "^1.5.5",
     "@types/react": "^19.1.10",
     "@types/react-dom": "^19.1.7",