Moves the cost summary out of a collapsible section below the tabs and into the Activity tab content, displayed as a static card at the top. Removes the now-unused `cost` state from `secondaryOpen`. Closes PAP-559 Co-Authored-By: Paperclip <noreply@paperclip.ing> Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>