feat(editor): add Prism grammar for Source-engine .cfg syntax

Five token classes (comment, string, keyword, number, identifier) plus
operators. Purely visual highlighting; no semantic validation of cvar
names or values.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

l4d2web/l4d2web/static/js/srccfg-grammar.js

@@ -0,0 +1,17 @@
+// Prism grammar for Source-engine config files (server.cfg-style).
+// Tokens: comment, string, number, keyword, identifier. Purely visual —
+// no semantic validation of cvars or values.
+(function (Prism) {
+  if (!Prism) return;
+  Prism.languages.srccfg = {
+    comment: /\/\/.*/,
+    string: {
+      pattern: /"(?:\\.|[^"\\])*"/,
+      greedy: true,
+    },
+    keyword: /\b(?:exec|alias|bind|unbind|toggle)\b/,
+    number: /\b-?\d+(?:\.\d+)?\b/,
+    identifier: /\b[a-zA-Z_][a-zA-Z0-9_]*\b/,
+    operator: /[+\-;]/,
+  };
+})(typeof window !== "undefined" ? window.Prism : undefined);