feat(ui): step by index, and give thread stepping a second binding

next_thread and prev_thread now walk with indexBelow/indexAbove, skipping
message rows, so they keep meaning thread-to-thread whatever is expanded.
Stepping message-to-message needs no code: QTreeView's own Up/Down walk VISIBLE
rows and already enter an expanded thread, and being the view's key handling
rather than a shortcut they stay inert when the message pane, a menu or an entry
bar has focus.

Item 60 turns out to have been fixed already, in 5487d58 on this branch, by
threadRowOf() walking up to the containing thread before doing the arithmetic.
The backlog entry was written against master, where that helper does not exist,
so it described a defect this branch had resolved a commit earlier. Verified by
writing both failing tests first and watching them pass: from the last reply of
an expanded thread, and from a thread root with its replies showing. They are
kept, because the property they assert is the one this change must not lose.
What the rewrite buys is that nothing is keyed on a row number any more, which
is the rule a deeper tree would break next.

Alt+Up/Down added alongside Ctrl+J/K. That required KeyMap::sequencesFor and a
move from setShortcut to setShortcuts, because the singular setter keeps only
the last binding and the second one was silently unreachable. Alt because
Shift+arrows is the built-in extend-selection that multi-row tagging depends on,
and because a bare arrow cannot be a window shortcut without breaking every text
field in the window, as Return already demonstrated. sequencesFor puts
sequenceFor's own choice first so the menus advertise an unchanged binding, and
sorts the tail, since QHash order is unspecified.

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