Open a thread on its latest message

[dhh]

A thread in the TUI used to open at its top, so on a long conversation the reader had to scroll past everything they'd already read to reach the reply that brought them there.

• The change: a thread now opens with its latest message's header at the top of the screen (showLatestEntry in internal/tui/mail.go). k steps back through earlier messages and j forward again. If the last message is short, the end of the one above it shows too, because the viewport can't scroll past the end of the thread.
• Why the latest and not the first unread: nothing HEY serves says which messages in a thread the reader hasn't seen. Entries carry no read state, and the posting's seen covers the thread as a whole. The latest message is where new mail arrives. Opening on the first unread would need HEY and the SDK to expose per-entry read state.

docs/tui.md gets one line. A new test opens a three-message thread through topicLoadedMsg and checks it lands on the last message, with k going back one.

🤖 Generated with Claude Code

[Copilot]

Copilot review overview

🟢 Approved

The focused implementation, tests, and documentation consistently support the intended navigation behavior.

Review effort: Balanced
Findings: None

What changed in this PR

Updates the TUI to open threads at their newest message, reducing unnecessary scrolling.

Changes:

• Positions the thread viewport at the latest entry.
• Tests latest-entry opening and backward navigation.
• Documents j/k message navigation.

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File	Description
internal/​tui/​mail.go	Opens threads at the newest entry offset.
internal/​tui/​mail_test.go	Verifies initial position and k navigation.
docs/​tui.md	Documents the updated behavior.

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