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/src/components/code/useWorkerHighlightedLines.ts

@@ -0,0 +1,26 @@
+import React from 'react';
+import { highlightLinesInWorker } from '@/components/chat/markdown/markdown-worker';
+
+// Tokenize a whole block ONCE in the Shiki worker and expose per-line inner
+// HTML. For per-line layouts (diffs, gutters, virtualization) that would
+// otherwise spawn one highlighter per row. Returns `null` until the first
+// result lands (or permanently on failure) — callers render plain text then.
+//
+// Whole-block tokenization also restores cross-line syntax context (multi-line
+// strings / comments) that independent per-line highlighting loses.
+export const useWorkerHighlightedLines = (code: string, language: string): string[] | null => {
+  const [lines, setLines] = React.useState<string[] | null>(null);
+
+  React.useEffect(() => {
+    let active = true;
+    setLines(null);
+    void highlightLinesInWorker(code, (language || 'text').toLowerCase()).then((result) => {
+      if (active) setLines(result);
+    });
+    return () => {
+      active = false;
+    };
+  }, [code, language]);
+
+  return lines;
+};