katex: Add CSS support for overline and underline borders.

Adds parsing and styling support for the overline-line and
underline-line CSS classes used in KaTeX rendering. These classes
apply a solid border-bottom-style to create the visual line effect.

The implementation adds:
- KatexSpanBorderBottomStyle enum for border style values
- borderBottomStyle and borderBottomWidthEm fields to KatexSpanStyles
- Parsing of overline-line/underline-line classes from KaTeX HTML
- Support for border-bottom-width inline style property

The overline and underline base classes are now explicitly ignored
in parsing, as they don't have CSS definitions in katex.scss but
appear in the generated HTML for semantic purposes.

Author: MdSaifAliMolla

lib/model/katex.dart

@@ -411,6 +411,7 @@ class _KatexParser {
     KatexSpanFontWeight? fontWeight;
     KatexSpanFontStyle? fontStyle;
     KatexSpanTextAlign? textAlign;
+    KatexSpanBorderBottomStyle? borderBottomStyle;
     var index = 0;
     while (index < spanClasses.length) {
       final spanClass = spanClasses[index++];
@@ -626,6 +627,8 @@ class _KatexParser {
         case 'nobreak':
         case 'allowbreak':
         case 'mathdefault':
+        case 'overline':
+        case 'underline':
           // Ignore these classes because they don't have a CSS definition
           // in katex.scss, but we encounter them in the generated HTML.
           // (Why are they there if they're not used?  The story seems to be:
@@ -636,6 +639,14 @@ class _KatexParser {
           // )
           break;
 
+        case 'overline-line':
+        case 'underline-line':
+          // .underline-line { width: 100%; border-bottom-style: solid; }
+          // Border applied via inline style: border-bottom-width: 0.04em;
+          widthEm = double.infinity;
+          borderBottomStyle = KatexSpanBorderBottomStyle.solid;
+          break;
+
         default:
           assert(debugLog('KaTeX: Unsupported CSS class: $spanClass'));
           unsupportedCssClasses.add(spanClass);
@@ -657,6 +668,8 @@ class _KatexParser {
       marginRightEm: _takeStyleEm(inlineStyles, 'margin-right'),
       color: _takeStyleColor(inlineStyles, 'color'),
       position: _takeStylePosition(inlineStyles, 'position'),
+      borderBottomStyle: borderBottomStyle,
+      borderBottomWidthEm: _takeStyleEm(inlineStyles, 'border-bottom-width')
       // TODO handle more CSS properties
     );
     if (inlineStyles != null && inlineStyles.isNotEmpty) {
@@ -840,6 +853,10 @@ enum KatexSpanPosition {
   relative,
 }
 
+enum KatexSpanBorderBottomStyle {
+  solid,
+}
+
 class KatexSpanColor {
   const KatexSpanColor(this.r, this.g, this.b, this.a);
 
@@ -893,6 +910,8 @@ class KatexSpanStyles {
 
   final KatexSpanColor? color;
   final KatexSpanPosition? position;
+  final KatexSpanBorderBottomStyle? borderBottomStyle;
+  final double? borderBottomWidthEm;
 
   const KatexSpanStyles({
     this.widthEm,
@@ -907,6 +926,8 @@ class KatexSpanStyles {
     this.textAlign,
     this.color,
     this.position,
+    this.borderBottomStyle,
+    this.borderBottomWidthEm,
   });
 
   @override
@@ -924,6 +945,8 @@ class KatexSpanStyles {
     textAlign,
     color,
     position,
+    borderBottomStyle,
+    borderBottomWidthEm
   );
 
   @override
@@ -940,7 +963,9 @@ class KatexSpanStyles {
       other.fontStyle == fontStyle &&
       other.textAlign == textAlign &&
       other.color == color &&
-      other.position == position;
+      other.position == position &&
+      other.borderBottomStyle == borderBottomStyle &&
+      other.borderBottomWidthEm == borderBottomWidthEm;
   }
 
   @override
@@ -958,6 +983,8 @@ class KatexSpanStyles {
     if (textAlign != null) args.add('textAlign: $textAlign');
     if (color != null) args.add('color: $color');
     if (position != null) args.add('position: $position');
+    if (borderBottomStyle != null) args.add('borderBottomStyle: $borderBottomStyle');
+    if (borderBottomWidthEm != null) args.add('borderBottomWidthEm: $borderBottomWidthEm');
     return '${objectRuntimeType(this, 'KatexSpanStyles')}(${args.join(', ')})';
   }
 
@@ -975,6 +1002,8 @@ class KatexSpanStyles {
     bool textAlign = true,
     bool color = true,
     bool position = true,
+    bool borderBottomStyle = true,
+    bool borderBottomWidthEm = true,
   }) {
     return KatexSpanStyles(
       widthEm: widthEm ? this.widthEm : null,
@@ -989,6 +1018,8 @@ class KatexSpanStyles {
       textAlign: textAlign ? this.textAlign : null,
       color: color ? this.color : null,
       position: position ? this.position : null,
+      borderBottomStyle: borderBottomStyle ? this.borderBottomStyle : null,
+      borderBottomWidthEm: borderBottomWidthEm ? this.borderBottomWidthEm : null,
     );
   }
 }