feat(ui): scope actions to the selected row kind and name it

tagSelected resolved rows to threads with threadAt(index.row()), which is wrong
for a message row: a child's row number indexes its siblings, so acting on a
reply tagged whichever thread sat at that position in the list. It now routes
through ThreadListModel::scopeFor, and a message row's change is sent as message
ids down applyTags with its own MessageTagCommand for undo.

MessageTagCommand stores message ids where ThreadTagCommand stores thread ids,
and that difference is the point rather than an inconsistency: re-resolving the
thread on undo would restore tags across every sibling the action never touched.

sendMessageTagChange deliberately skips the optimistic model update.
applyTagChange is keyed by thread and would repaint the whole row as though
every message in it had changed, which for a one-message edit is a lie the user
watches correct itself on the next query. It keeps the two things that are NOT
optional: the edited-account set, resolved through the containing thread since
the account is a property of the thread, and holding the edit when a sync holds
notmuch's write lock, since the worker's read-write open blocks rather than
failing.

The scope is now stated before an action and after it, naming both the message
count and whether a whole thread went. This is what stands in for the
confirmation dialog CLAUDE.md rules out: undo is the safety net, and undo is
only usable if the user can tell that something larger than they meant has just
happened. Selecting a single message reports no count at all, since reading one
message is not a bulk action.

A mutation that routed message rows down the thread path SURVIVED the whole
suite: undo depth and status text are identical either way while every sibling
gets tagged. anActionOnAMessageRowTagsThatMessageNotTheThread exists because
that gap was found, and asserts on the ids actually sent.

anActionOnAThreadRowSaysItHitTheWholeThread reads the status bar BEFORE draining
the event loop. This binary has no worker, backlog item 36, so the queued write
reaches a database that has never heard of the thread and answers with
errorOccurred, which overwrites the status bar: draining first asserts on that
error and fails against correct code.

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