refactor: remove flutter_highlight dependency and enhance code block styling

- Removed flutter_highlight as a dependency to streamline the codebase.
- Updated CodeBlockHeader to improve styling based on the current theme, enhancing visual consistency.
- Adjusted code block background and text colors to match GitHub/Atom themes for better readability.
- Refactored markdown processing logic to eliminate reliance on deprecated highlight features, improving maintainability.

lib/shared/widgets/markdown/code_block_header.dart

@@ -14,16 +14,30 @@ class CodeBlockHeader extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     final theme = context.conduitTheme;
-    final label = language.isEmpty ? 'code' : language;
+    final materialTheme = Theme.of(context);
+    final isDark = materialTheme.brightness == Brightness.dark;
+    final label = language.isEmpty ? 'plaintext' : language;
+
+    // Match GitHub/Atom theme colors
+    final backgroundColor = isDark
+        ? const Color(0xFF282c34) // Atom One Dark header
+        : const Color(0xFFf6f8fa); // GitHub light header
+    final textColor = isDark
+        ? const Color(0xFF9da5b4) // Muted text for dark
+        : const Color(0xFF57606a); // GitHub gray for light
+
     return Container(
       padding: const EdgeInsets.symmetric(
-        horizontal: Spacing.sm,
+        horizontal: Spacing.md,
         vertical: Spacing.xs,
       ),
       decoration: BoxDecoration(
-        color: theme.surfaceContainer.withValues(alpha: 0.35),
-        borderRadius: const BorderRadius.vertical(
-          top: Radius.circular(AppBorderRadius.sm),
+        color: backgroundColor,
+        border: Border(
+          bottom: BorderSide(
+            color: theme.cardBorder.withValues(alpha: 0.15),
+            width: 1,
+          ),
         ),
       ),
       child: Row(
@@ -31,16 +45,26 @@ class CodeBlockHeader extends StatelessWidget {
           Text(
             label,
             style: AppTypography.codeStyle.copyWith(
-              color: theme.textSecondary,
-              fontWeight: FontWeight.w600,
+              color: textColor,
+              fontSize: 12,
+              fontWeight: FontWeight.w500,
             ),
           ),
           const Spacer(),
-          IconButton(
-            icon: const Icon(Icons.copy_rounded, size: 18),
-            color: theme.iconPrimary,
-            tooltip: 'Copy code',
-            onPressed: onCopy,
+          Material(
+            color: Colors.transparent,
+            child: InkWell(
+              onTap: onCopy,
+              borderRadius: BorderRadius.circular(4),
+              child: Padding(
+                padding: const EdgeInsets.all(6),
+                child: Icon(
+                  Icons.content_copy_rounded,
+                  size: 16,
+                  color: textColor,
+                ),
+              ),
+            ),
           ),
         ],
       ),