Messages: full thread view + batched reply counts #35

Product behaviour

Replies nest under the parent. When a reply has sub-replies a "+N replies" link appears, opening the full thread view: the original post at the top and every reply chained below, each with its own Reply and Dig buttons.

The app shows replies inline on a card but has no dedicated thread view and no nested-reply affordance.

API

• GET /api/messages/{id}/replies (already implemented).
• POST /api/messages/{id}/reply-counts — not implemented; batch reply counts for a set of messages.
• The live payload already carries replyCounts, replyCount, _count and repliesCheckedAt on each message — none are modelled.

Acceptance criteria

• Models/Message.cs gains ReplyCounts, ReplyCount, RepliesCheckedAt.
• PostReplyCountsAsync(ids) added to Services/InterlinedApiClient.Messages.cs.
• +N replies affordance on cards with sub-replies.
• A thread view: original at top, replies chained below, each with Reply and Dig.
• Reachable both from +N replies and from clicking a card's body.