feat(web): focused Notion-like brief writing overlay

[AlexsanderHamir]

Summary

• PromptFocusFrame expands the compose brief with position: fixed so the same TipTap instance keeps caret and HTML.
• Overlay chrome is “Editing brief”, word count, Done; Escape closes; grip hides while expanded.
• inert on compose chrome; --z-prompt-focus: 60 sits under mention/slash popovers.

Test plan

• Expand opens a dialog; Done and Escape restore focus to Expand.
• Typing in the editor survives expand/collapse (no remount).
• Mentions/slash/bubble still appear above the overlay.