Native toolbar mobile support

[mbarta]

This PR adds native Strada bridge support code for Lexxy's toolbar, selection, and attachments so native Android/iOS apps can render their own toolbar UI while the editor handles the content.

• Attribute syncing: Extracts a Native class that emits attributes-change events on every editor state update and focus, reporting active formatting (bold, italic, highlight, link, etc.) so the native toolbar can reflect the current selection state
• Selection freeze/thaw: Adds freezeSelection()/thawSelection() on the editor element so native dialogs (link editing, etc.) can preserve the user's selection while a modal is open. Expands collapsed selections within links to select the full link text before freezing
• Highlight colors: Dispatches resolved highlight color palettes (highlight-colors event) on initialization so the native side knows which swatches to display, including computed RGB values for CSS custom properties
• Code language picker: Intercepts the native mousedown to emit a code-language-picker-open event, letting native apps show their own picker UI. Adds setCodeLanguage command for applying the selection back
• Bridge-managed attachments: insertPendingAttachment() creates upload nodes without a uploadUrl, skipping DirectUpload. Returns a handle with setAttributes(), setUploadProgress(), and remove() for the native side to drive the upload lifecycle
• Disable Autocapitalize: prevents mobile keyboards from capitalizing input

This support functionality needs to be paired with a Strada controller and native mobile implementation.