Add enhanced horizontal rule extension with markdown indentation support

- Create EnhancedHorizontalRule extension that preserves leading spaces (0-3) in markdown serialization
- Store indentation as data attribute and restore it during markdown rendering
- Fix YAML front matter parsing to not match indented horizontal rules as front matter

src/renderer/src/components/RichEditor/extensions/enhanced-horizontal-rule.ts

@@ -0,0 +1,74 @@
+import { mergeAttributes } from '@tiptap/core'
+import { HorizontalRule } from '@tiptap/extension-horizontal-rule'
+
+/**
+ * Enhanced HorizontalRule extension that preserves leading spaces in markdown
+ *
+ * Standard Markdown allows 0-3 spaces before a horizontal rule (---, ***, ___)
+ * This extension preserves that indentation when serializing back to markdown
+ */
+export const EnhancedHorizontalRule = HorizontalRule.extend({
+  addAttributes() {
+    return {
+      ...this.parent?.(),
+      // Store the number of leading spaces (0-3)
+      indentation: {
+        default: 0,
+        parseHTML: (element) => {
+          const indent = element.getAttribute('data-indentation')
+          return indent ? parseInt(indent, 10) : 0
+        },
+        renderHTML: (attributes) => {
+          if (!attributes.indentation) return {}
+          return {
+            'data-indentation': attributes.indentation
+          }
+        }
+      }
+    }
+  },
+
+  parseHTML() {
+    return [
+      {
+        tag: 'hr',
+        getAttrs: (element) => {
+          if (typeof element === 'string') return {}
+
+          const htmlElement = element as HTMLElement
+          const indentAttr = htmlElement.getAttribute('data-indentation')
+
+          return {
+            indentation: indentAttr ? parseInt(indentAttr, 10) : 0
+          }
+        }
+      }
+    ]
+  },
+
+  renderHTML({ HTMLAttributes }) {
+    return ['hr', mergeAttributes(this.options.HTMLAttributes, HTMLAttributes)]
+  },
+
+  // Custom markdown parsing to capture leading spaces
+  parseMarkdown(token) {
+    // The token.raw contains the original markdown including leading spaces
+    // Match 0-3 leading spaces before ---, ***, or ___
+    const match = /^( {0,3})(?:---|___|\*\*\*)/.exec(token.raw || '')
+    const indentation = match ? match[1].length : 0
+
+    return {
+      type: this.name,
+      attrs: {
+        indentation
+      }
+    }
+  },
+
+  // Custom markdown serialization to restore leading spaces
+  renderMarkdown(node) {
+    const indentation = node.attrs?.indentation || 0
+    const spaces = ' '.repeat(Math.min(indentation, 3)) // Max 3 spaces per spec
+    return spaces + '---\n\n'
+  }
+})

src/renderer/src/components/RichEditor/extensions/yaml-front-matter.ts

@@ -23,7 +23,7 @@ export const YamlFrontMatter = Node.create({
     level: 'block',
 
     start(src: string) {
-      const result = src.match(/^\s*---\n/) ? 0 : -1
+      const result = src.match(/^---\n/) ? 0 : -1
       return result
     },
     // Parse YAML front matter