Messages: full thread view and reply counts #25

Part of #17 — Message composer & feed parity gaps

Web behaviour: replies nest under their parent, and when a reply has sub-replies a
"+N replies" link opens the full thread view — the original post at the top with every reply
chained below, each with its own Reply and Dig buttons.

Android has GET /api/messages/{id}/replies and a detail screen, but no nested "+N replies"
affordance and no reply counts.

• POST /api/messages/{id}/reply-counts (unused today) supplies the counts.
• Render the "+N replies" link and the chained thread view.

Tests: counts parse; link navigates to the full thread.

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