fix(editor): resolve HeadingTitle shift/reduce conflict in Typst grammar

headingTitleItem* allowed an empty HeadingTitle, causing a shift/reduce
conflict: after HeadingMark, seeing "*" the LR parser couldn't decide
whether to shift it as a Strong inside the heading or reduce HeadingTitle
to empty and treat "*" as a document-level item.

Changing to headingTitleItem+ forces HeadingTitle to be non-empty, so
"*" after HeadingMark must be inside the heading. Empty headings are
handled by Lezer's error recovery.

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

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

@@ -31,7 +31,10 @@ item {
 // HeadingMark is produced by an external tokenizer that enforces the
 // start-of-line constraint and captures the "=+" prefix + trailing space.
 Heading { HeadingMark HeadingTitle }
-HeadingTitle { headingTitleItem* }
+// headingTitleItem+ (not *) so HeadingTitle is never empty: the generator
+// would otherwise produce a shift/reduce conflict on "*" after HeadingMark,
+// unable to tell if "*" starts a Strong inside the heading or at doc level.
+HeadingTitle { headingTitleItem+ }
 headingTitleItem {
   Strong | Emphasis | CodeExpr | InlineMath | RawInline | Label | Ref | HeadingText
 }