fix(typst): fix zero-length token error for LineCommentContent

LineCommentContent { ![\n]* } matches the empty string, which Lezer
rejects as a zero-length token (infinite-loop risk). Change to ![\n]+
and mark it optional in the LineComment rule so empty // comments parse.

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

services/web/frontend/js/features/source-editor/lezer-typst/typst.grammar

@@ -38,7 +38,7 @@ Heading { HeadingMark HeadingTitle? }
 // External tokens are context-isolated by the LR state machine — no merging.
 
 // ── Comments ──────────────────────────────────────────────────────────────
-LineComment { "//" LineCommentContent }
+LineComment { "//" LineCommentContent? }
 // BlockCommentBody is external so it can track nesting depth —
 // Typst supports /* /* nested */ */ block comments.
 BlockComment { "/*" BlockCommentBody? "*/" }
@@ -182,7 +182,7 @@ Escape { "\\" EscapeChar }
   }
 
   // Comment content — everything to end of line.
-  LineCommentContent { ![\n]* }
+  LineCommentContent { ![\n]+ }
 
   // Math content — everything between the $ delimiters (no crossing newlines).
   MathContent { ![$\n]+ }