Support absolute paths in setHTMLBody for automatic CID embedding

[Kaczmi]

• bug fix / new feature? not sure
• BC break? yes

Currently, setHTMLBody in Nette\Mail supports a basePath for automatically embedding local images as CID attachments. Relative paths like img/logo.svg work correctly, but absolute paths starting with a slash (e.g. /img/logo.svg) are ignored and not embedded.

This PR updates the logic so that images referenced with a leading slash are correctly resolved relative to the provided basePath, allowing projects that use Webpack-generated assets (which always output to a fixed public folder with leading slashes) to work without breaking existing templates that rely on relative paths.

Here, basePath for mails is intended to represent the www folder of the application, making it clear that absolute paths are resolved relative to the web root.

[dg]

Thanks for the PR, but I'd like to pass on this one.

The title says „absolute paths", but the change actually reinterprets root-relative paths (/foo.png) as relative to $basePath. That only works when $basePath equals the document root and images live directly under it. I'd prefer to keep setHtmlBody predictable: relative paths are resolved against $basePath, everything else is left alone.

If you need root-relative URLs from templates to be embedded, a cleaner approach would be an explicit opt-in, e.g. a URL prefix that gets stripped before resolving against $basePath. That keeps the semantics of / clear and gives the caller a real knob for URL-to-filesystem mapping.