feat(editor-v2): light + dark themes + syntax highlight style

themes.js exports four extensions:
- editorLightTheme / editorDarkTheme: EditorView.theme() variants
  keyed to the --cm-* CSS variables defined in tokens.css (light) and
  its prefers-color-scheme: dark block.
- editorHighlightStyle: HighlightStyle bound to Lezer tags
  (comment, string, number, keyword, variableName).
- editorHighlighting: syntaxHighlighting(editorHighlightStyle) ready
  to drop into the EditorState extensions array.

@lezer/highlight comes in transitively via @codemirror/language;
no new package.json dependency needed.

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

l4d2web/scripts/editor-src/themes.js

@@ -0,0 +1,49 @@
+import { EditorView } from "@codemirror/view";
+import { HighlightStyle, syntaxHighlighting } from "@codemirror/language";
+import { tags as t } from "@lezer/highlight";
+
+// CSS variables are defined in static/css/tokens.css (light) and the
+// `prefers-color-scheme: dark` block. Both themes route through the
+// same --cm-* variable names; the OS toggle swaps the underlying values.
+//
+// Two named themes so we can also force-pick light/dark in tests if needed.
+
+const baseRules = {
+  "&": {
+    backgroundColor: "var(--cm-bg)",
+    color: "var(--cm-fg)",
+    fontFamily: "var(--font-mono, ui-monospace, SFMono-Regular, Menlo, monospace)",
+    fontSize: "14px",
+  },
+  ".cm-content": { caretColor: "var(--cm-fg)", padding: "8px" },
+  ".cm-cursor": { borderLeftColor: "var(--cm-fg)" },
+  "&.cm-focused .cm-selectionBackground, .cm-selectionBackground, ::selection": {
+    backgroundColor: "var(--cm-selection)",
+  },
+  ".cm-gutters": {
+    backgroundColor: "var(--cm-bg)",
+    color: "var(--fg-muted, #888)",
+    border: "none",
+  },
+  ".cm-tooltip": {
+    backgroundColor: "var(--cm-bg)",
+    border: "1px solid var(--border-strong, #444)",
+    color: "var(--cm-fg)",
+  },
+  ".cm-tooltip-autocomplete > ul > li[aria-selected]": {
+    backgroundColor: "var(--cm-selection)",
+  },
+};
+
+export const editorLightTheme = EditorView.theme(baseRules, { dark: false });
+export const editorDarkTheme = EditorView.theme(baseRules, { dark: true });
+
+export const editorHighlightStyle = HighlightStyle.define([
+  { tag: t.comment, color: "var(--cm-comment)" },
+  { tag: t.string, color: "var(--cm-string)" },
+  { tag: t.number, color: "var(--cm-number)" },
+  { tag: t.keyword, color: "var(--cm-keyword)" },
+  { tag: t.variableName, color: "var(--cm-fg)" },
+]);
+
+export const editorHighlighting = syntaxHighlighting(editorHighlightStyle);