TUI: trash every selected thread with t

[bulters]

What

Space already selects threads in the TUI, but only Ctrl+B (bulk reply) acted on the selection. t and the other filing keys took the row under the cursor, so trashing a handful of threads meant one keypress per row.

With a selection, t now trashes every selected thread in a single POST /postings/trash (the SDK's MoveToTrash is already variadic), the way the web app's toolbar acts on a selection. Without a selection it behaves exactly as before.

How

• postingActionDoneMsg gains postingIDs, every posting a bulk action took. The box list, the Previously Seen screen and the overlaid search/bundle lists iterate it, falling back to the single postingID for every existing action.
• contentList.removeAt now drops the removed row from the selection. A successful bulk trash therefore clears the selection by itself, and a failed request leaves it standing for another try.
• docs/tui.md notes that t acts on the selection when there is one.

Related: #330 asks for the same over seen/unseen. This only wires t, since that was the itch; the same postingIDs path would carry the other keys.

Tests

• TestMailViewTrashesSelectedThreadsInOneRequest: two selected rows, one request with [100, 101], both rows gone, selection empty, toast reads "2 threads moved to Trash".
• TestMailViewTrashFailureKeepsSelection: a 500 leaves both rows and the selection in place.
• make check passes.

🤖 Generated with Claude Code

https://claude.ai/code/session_01LZgWZQHxm7KdZNLNjxCtQG

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Summary by cubic

Fixes the TUI so t trashes every selected thread in one request when there is a selection, instead of only the row under the cursor; without a selection it behaves exactly as before.

• A successful trash clears the selection; a failed request leaves it standing for another try.
• The done message now carries every posting a bulk action took, so the box list, Previously Seen screen, and overlaid search/bundle lists drop each affected row, and docs/tui.md notes the new behavior.
• Adds tests for the one-request bulk trash and for a failed request keeping the selection.
• Only t is wired; the same path can carry the other filing keys later.

^{Written for commit dcd5a45. Summary will update on new commits.}

[Copilot]

🟢 Approval recommended

The behavior is coherent and tested; the only finding is a non-blocking comment typo.

Pull request overview

Adds bulk trash support for selected TUI threads while preserving single-row behavior.

Changes:

• Sends selected posting IDs in one trash request.
• Removes successful results from lists and selections.
• Adds success/failure tests and updates TUI documentation.

[!TIP]
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File summaries

File	Description
internal/tui/mail.go	Implements bulk trash and multi-posting completion handling.
internal/tui/mail_test.go	Tests successful and failed bulk trash operations.
internal/tui/content.go	Prunes removed rows from selections.
docs/tui.md	Documents selection-aware trash behavior.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 1
• Review effort level: Balanced

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