Fix attachToolbar race under Turbo navigation

[finist]

Fix race condition when using custom toolbar with Turbo navigation

This PR fixes a race condition that occurs when <lexxy-editor> is initialized before the custom <lexxy-toolbar> is upgraded as a Web Component during Turbo-driven navigation.

Problem

When navigating with Turbo, DOM elements are inserted incrementally.
<lexxy-editor> often connects before the custom toolbar is upgraded, so:

this.toolbarElement.setEditor(this)

throws:

TypeError: this.toolbarElement.setEditor is not a function

This interrupts editor initialization and prevents content from loading.

Related issue:
#458

What this PR changes

• attachToolbar() no longer assumes that toolbarElement is already a fully upgraded custom element.
• The method now:

  • attempts to attach immediately (for full page loads),

  • but if setEditor is not yet available, it waits for:

    customElements.whenDefined("lexxy-toolbar")

    and retries on the next animation frame.

• When a custom toolbar is specified via the toolbar attribute, Lexxy no longer creates a default toolbar.
  This prevents the duplicate-toolbar issue under Turbo navigation.

Result

• Custom toolbars now work reliably with Turbo navigation.
• No errors are thrown during editor initialization.
• Editor content loads correctly.
• Default behavior (when no custom toolbar is provided) remains unchanged.

[zoltanhosszu]

@finist thank you for this! Could you move these changes to src/elements/toolbar.js and use the build commands to rebuild the source?

[finist]

@zoltanhosszu done