feat(rules): preview a rule's mail in the thread list

Item 77. The dialog could say how many messages a rule matched and not
which ones. A Preview in list button now runs the selected rule's query
in the main window; the dialog stays open, since comparing the rule
against its results is the point.

Two constraints from the backlog entry, both now asserted and both
mutation-checked.

The query runs exactly as stored, with no tag:new and no wrapping
parentheses. The post-new hook adds those when it applies a rule, and a
preview that copied them would match nothing outside a sync window,
since tag:new is set only on mail that has just arrived.

The account selector is cleared first. runQuery() wraps the bar's text
in the selected account's scope, and a rule query usually names its own
path already, so previewing one with an account selected would scope it
twice and show an empty list, which reads as "this rule collects no
mail".

The second mutation only fails once the test's config has an account to
select: with the default empty config the selector sits on "All
accounts" anyway, and asserting that a preview leaves it there passed
against the mutation. Recorded in the test.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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