[Excalidraw] Support for Mermaid import with line breaks

Hey there, I noticed that when importing Mermaid diagrams that use `<br>` tags for multi-line node labels, the `<br>` shows up as literal text in the rendered diagram instead of being converted to actual line breaks. ### Steps to reproduce 1. Paste or enter a Mermaid diagram using `<br>` in node labels: ``` graph TD A["User Registration<br>Process"] --> B["Validate Input<br>Data"] B --> C{"Is Data<br>Valid?"} C -->|Yes| D["Create User<br>Account"] ``` 2. The resulting Excalidraw elements show the raw `<br>` string in the text (e.g., `User Registration<br>Process`) instead of rendering it as two lines. ### Expected behavior The `<br>` tag should be converted to a newline so the text displays as: ``` User Registration Process ``` ### What's happening The `@excalidraw/mermaid-to-excalidraw` library returns skeleton elements where `text` and `label.text` still contain the literal `<br>` string. Excalidraw doesn't sanitize these before converting them to final elements