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/web/package.json

@@ -70,7 +70,6 @@
     "@types/node": "^24.3.1",
     "@types/react": "^19.1.10",
     "@types/react-dom": "^19.1.7",
-    "@types/react-syntax-highlighter": "^15.5.13",
     "@types/supertest": "^7.2.0",
     "@vitejs/plugin-react": "^5.0.0",
     "autoprefixer": "^10.4.21",
@@ -90,7 +89,6 @@
     "react": "^19.1.1",
     "react-dom": "^19.1.1",
     "react-markdown": "^10.1.0",
-    "react-syntax-highlighter": "^15.6.6",
     "remark-gfm": "^4.0.1",
     "sonner": "^2.0.7",
     "strip-json-comments": "^5.0.3",