Scheduled posts: edit content, time and destinations (today only cancel is possible) #23

Part of #17 — Message composer & feed parity gaps

Web behaviour: each scheduled post shows its content, scheduled time and selected
destinations, and an edit affordance changes the time, the message content, or which networks
receive it — including the LinkedIn destination picker. Deleting cancels it.

ScheduledMessagesScreen only offers onCancel.

• Add editing of content, scheduledAt and cross-post targets to the scheduled list.
• The scheduling dialog itself should offer a checkbox per connected cross-post account, as the
  web one does.

Tests: edit round-trip; destination changes persist.

Filed from the parity review of feature/parity-wip @ 14d7795 against interlinedlist.com (OpenAPI /api/openapi.json, 233 paths) and the public help centre.