feat(editor): Mermaid syntax highlighting, linter and autocomplete

- StreamLanguage tokenizer for Mermaid: diagram-type keywords, general
  keywords, directions, arrows/links, strings, %% comments, init
  directives and YAML frontmatter, with a dark-theme HighlightStyle
- live linter via mermaid.parse: maps parse errors to the right editor
  line (accounting for the frontmatter lines mermaid strips), with wavy
  underline, gutter marker and message tooltip
- autocomplete: Mermaid keyword/type/direction options plus starter
  diagram snippets offered on the first line; bracket closing
- dark-themed completion + diagnostic tooltips

Adds @codemirror/lint, @codemirror/autocomplete and @lezer/highlight.

package.json

@@ -13,10 +13,13 @@
     "app:build": "tauri build"
   },
   "dependencies": {
+    "@codemirror/autocomplete": "^6.18.0",
     "@codemirror/commands": "^6.6.0",
     "@codemirror/language": "^6.10.2",
+    "@codemirror/lint": "^6.8.0",
     "@codemirror/state": "^6.4.1",
     "@codemirror/view": "^6.28.0",
+    "@lezer/highlight": "^1.2.0",
     "@tauri-apps/api": "^2.0.0",
     "@tauri-apps/plugin-dialog": "^2.0.0",
     "codemirror": "^6.0.1",