feat(editor): Shiki syntax highlighting in the file editor (CodeMirror)

Bring the CodeMirror file editor up to the same rich highlighting as the Shiki
file view, so toggling edit <-> view is visually consistent. lezer collapses far
more tokens than TextMate (import/from/const are all "keyword"), so a theme
remap can't reach parity — instead, project real Shiki tokens onto decorations.

- Worker: add highlightTokens — tokenize with an arbitrary registered TextMate
  theme and return per-line styled runs with offsets. The theme object ships to
  the worker once per name; later calls send only the name.
- New shikiHighlight CodeMirror extension: a StateField of mark decorations
  built from worker tokens. Re-tokenizes on a short idle (off the keystroke
  path) and maps decorations through edits so colors persist while typing.
- flexokiTheme: add { syntaxColors: false } to keep only the editor UI theme,
  so the lezer highlighter doesn't compete with the Shiki decorations.
- FilesView: enable Shiki highlighting (same language resolver as the file view
  → identical language) and drop lezer token colors when it's active. lezer
  language stays on for indentation/folding/brackets.

packages/ui/src/components/chat/markdown/markdown-worker.ts

@@ -1,5 +1,5 @@
 import MarkdownShikiWorkerUrl from './markdown-shiki.worker.ts?worker&url';
-import type { MarkdownWorkerRequest, MarkdownWorkerResponse } from './markdown-worker-protocol';
+import type { MarkdownTokenRun, MarkdownWorkerRequest, MarkdownWorkerResponse } from './markdown-worker-protocol';
 
 // Main-thread client for the markdown Shiki worker. Moves syntax tokenization
 // off the UI thread: a closed code block is shipped to the worker, which returns
@@ -12,10 +12,14 @@ type PendingResolver = (response: MarkdownWorkerResponse | null) => void;
 let worker: Worker | undefined;
 let nextId = 0;
 const pending = new Map<number, PendingResolver>();
+// Theme names whose full definition we've already shipped to the live worker, so
+// repeat tokenization sends only the name (not the whole theme object) again.
+const sentThemes = new Set<string>();
 
 const failAll = (): void => {
   pending.forEach((resolve) => resolve(null));
   pending.clear();
+  sentThemes.clear();
   worker?.terminate();
   worker = undefined;
 };
@@ -68,3 +72,31 @@ export const highlightLinesInWorker = async (code: string, lang: string): Promis
   const response = await request((id) => ({ type: 'highlightLines', id, code, lang }));
   return response?.type === 'highlightLines' ? response.lines : null;
 };
+
+/**
+ * Tokenize `code` with the given resolved TextMate theme and return per-line
+ * styled runs with offsets — for building CodeMirror decorations that match the
+ * Shiki file view exactly. The full theme object is shipped only the first time
+ * a theme name is seen by the live worker. Resolves to `null` on failure.
+ */
+export const highlightTokensInWorker = async (
+  code: string,
+  lang: string,
+  themeName: string,
+  theme: unknown,
+): Promise<MarkdownTokenRun[][] | null> => {
+  const needsTheme = !sentThemes.has(themeName);
+  const response = await request((id) => ({
+    type: 'highlightTokens',
+    id,
+    code,
+    lang,
+    themeName,
+    ...(needsTheme ? { theme } : {}),
+  }));
+  if (response?.type === 'highlightTokens') {
+    sentThemes.add(themeName);
+    return response.lines;
+  }
+  return null;
+};