fix(v0.1.14): disk file system, syntax highlight transparency, zero-dep Mermaid

fix(plugins): rewrite fileSystem to use File System Access API
- openFile(): showOpenFilePicker for real .md disk files
- saveFile(): showSaveFilePicker or write to existing handle
- saveFileAs(): force save-as dialog
- Graceful fallback when API unsupported

fix(core): syntax highlight layer transparency
- textarea gets me-textarea-highlight class when syntaxHighlight enabled
- CSS: color transparent, caret-color visible, selection bg visible
- Highlight layer positioned after gutter (left:36px)

verify(parser): Mermaid self-implemented container
- Creates <div class="me-mermaid"> with <pre class="mermaid">
- Zero third-party dependencies required
- Users optionally load Mermaid.js to initialize rendering

src/core.js

@@ -229,6 +229,9 @@ class MarkdownEditor {
     textarea.style.tabSize = String(this.config.tabSize || 2);
     textarea.placeholder = this.config.placeholder || i18nT('placeholder');
     textarea.setAttribute('aria-label', i18nT('edit'));
+    if (this.config.syntaxHighlight) {
+      textarea.classList.add('me-textarea-highlight');
+    }
     editorInner.appendChild(textarea);
 
     editorPane.appendChild(editorInner);