Show document connection status in the document header

Move PlanStatus out of the floating bottom-right pill into the document
pane's header row (a row of its own on phones). Reconnecting is a quiet
warning dot and label; a connection lost for five seconds, a refusal, or
an open failure becomes a destructive label with a Reload action and the
detail in a tooltip. The editor dims while offline and the chat composer
says it is reconnecting.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>