dashed border and "Inbox" label. Auto-synced media slides in with
spring animations and random offset/rotation for a natural look.
MattermostImport: modal dialog for pulling images from a MM thread
URL and managing linked channels for auto-sync. Uses the existing
dark modal styling pattern from ShortcutsHelp.
Editor wiring: toolbar button for MM import, InboxZone added to
viewport on init, socket listener for board:media-arrived events
that routes assets into the InboxZone with a toast notification.