feat: 優化代碼高亮系統並修復樣式問題 (v5.5.4-b1)

- 簡化 config.pug 中的高亮設置檢測邏輯
- 重構滾動條樣式，提取為可重用的 $scrollbar-style
- 優化 JavaScript 代碼高亮邏輯，增加對默認高亮的支持
- 修復打賞彈窗的邊框圓角應用位置
- 改善代碼高亮工具的檢測和處理流程

layout/includes/head/config.pug

@@ -72,20 +72,16 @@
     })
   }
 
-  let highlight = 'undefined'
-  let syntaxHighlighter = config.syntax_highlighter
-  let highlightEnable = syntaxHighlighter ? ['highlight.js', 'prismjs'].includes(syntaxHighlighter) : (config.highlight.enable || config.prismjs.enable)
-  if (highlightEnable) {
-    const { copy, language, height_limit, fullpage, macStyle } = theme.code_blocks
-    highlight = JSON.stringify({
-      plugin: syntaxHighlighter ? syntaxHighlighter : config.highlight.enable ? 'highlight.js' : 'prismjs',
-      highlightCopy: copy,
-      highlightLang: language,
-      highlightHeightLimit: height_limit,
-      highlightFullpage: fullpage,
-      highlightMacStyle: macStyle
-    })
-  }
+  let highlightProvider = config.syntax_highlighter || (config.highlight.enable ? 'highlight.js' : config.prismjs.enable ? 'prismjs' : null)
+  const { copy, language, height_limit, fullpage, macStyle, shrink } = theme.code_blocks
+  let highlight = JSON.stringify({
+    plugin: highlightProvider,
+    highlightCopy: copy,
+    highlightLang: language,
+    highlightHeightLimit: height_limit,
+    highlightFullpage: fullpage,
+    highlightMacStyle: macStyle
+  })
 
 script.
   const GLOBAL_CONFIG = {