Fix mermaid diagram rendering with YAML frontmatter

[gjouret]

Summary

Mermaid diagrams that use YAML frontmatter (--- config blocks) fail to render in preview — they display as plain text code blocks instead of graphical diagrams. This PR fixes three root causes:

• getPrettifiedContent() globally replaced \n---\n with \n<hr>\n, including inside fenced code blocks, destroying mermaid YAML frontmatter delimiters. Added replaceHorizontalRulesOutsideCodeBlocks() that uses regex to skip fenced code blocks.
• cleanMetaData() stripped all --- separators when a note has YAML front matter. components(separatedBy: "---") splits on every occurrence, and list.joined() rejoined without the separator. Fixed with list.joined(separator: "---").
• Mermaid v11 API and rendering improvements: Updated from deprecated mermaid.init() to async mermaid.run(), bundled @mermaid-js/layout-elk plugin for ELK layout support, added YAML frontmatter indentation normalization (tabs/mixed whitespace → consistent spaces), changed useMaxWidth to true so diagrams fit the preview width, and added visible error messages on render failure.

Example mermaid that now renders correctly

```mermaid
---
title: My Diagram
config:
  theme: forest
  layout: elk
---
flowchart LR
    A["Node A"] --> B["Node B"]
```

Test plan

• Create a note with a mermaid diagram that has YAML frontmatter (including layout: elk) — verify it renders as a graphical diagram
• Create a note with a simple mermaid diagram (no frontmatter) — verify it still renders
• Create a note with YAML front matter (---\ntitle: ...\n---) AND a mermaid diagram with its own --- frontmatter — verify both are handled correctly
• Verify complex diagrams fit within the preview width (no horizontal overflow)
• Verify that non-mermaid code blocks containing --- are unaffected

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