Create from…: "To Message" prefills the composer instead of creating #16

Part of #10 — "Create from…" (materialize) — turn messages, lists, rows and documents into new lists/documents/posts

To Message is the one destination that creates nothing by itself — it hands the user a
draft in the normal composer, where they edit it, add cross-post targets or a schedule, and post
it themselves.

• Route from any Create-from entry point into the composer with the generated draft prefilled.
• A message cannot be converted to a message (use Quote or Push for that) — hide the option
  there.
• Character-limit interaction: if the draft is longer than the limit the composer shows it as
  usual so the user can shorten it or let it post as a thread. When cross-post targets are
  selected the binding limit is the tightest one among them (Bluesky's 300 is usually it), not
  InterlinedList's own — see the composer epic.

Tests: prefill navigation carries the draft; option hidden on a message source.

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