[itscheems]

Mention completion menus now share a portaled popover and textarea caret measurement, keeping the existing card presentation while escaping clipped form and modal containers. Viewport collision handling keeps the menu usable in narrow and short layouts, and keyboard navigation keeps the selected option visible.

Field focus, pointer insertion, composition keys, blur dismissal, and Escape behavior are preserved across the shared writing controls. An open suggestion list handles Escape before its writing dialog closes. The superseded custom placement helper and its calculation-only tests are removed in favor of shared positioning and real-browser verification.

Validation:

• 180 tests across seven affected files passed, including eight completion interaction regressions.
• Web build/type checking, lint, and formatting passed.
• Browser checks covered pointer and keyboard insertion, modal ownership, focus retention, dismissal, and responsive placement.

Persistence and API contracts are unchanged.

Closes #215