Clearer labels on shared controls

A few labels that appear across the platform now read more plainly:

• The control that opens and closes the sidebar is announced to screen readers as “Toggle sidebar”.
• While the assistant prepares a reply or a board heading, the placeholder reads “Working…”.
• In the rich-text editor’s link dialog, the fields read “Display text (optional)” and “Open in new tab”.

Nothing else changes: every list, form and chat box keeps the wording you already see.