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.

pubspec.lock

@@ -430,14 +430,6 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "3.4.1"
-  flutter_highlight:
-    dependency: "direct main"
-    description:
-      name: flutter_highlight
-      sha256: "7b96333867aa07e122e245c033b8ad622e4e3a42a1a2372cbb098a2541d8782c"
-      url: "https://pub.dev"
-    source: hosted
-    version: "0.7.0"
   flutter_lints:
     dependency: "direct dev"
     description:
@@ -621,14 +613,6 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.3.2"
-  highlight:
-    dependency: transitive
-    description:
-      name: highlight
-      sha256: "5353a83ffe3e3eca7df0abfb72dcf3fa66cc56b953728e7113ad4ad88497cf21"
-      url: "https://pub.dev"
-    source: hosted
-    version: "0.7.0"
   hive_ce:
     dependency: "direct main"
     description: