feat(queries): make Sent a saved query rather than a fixed button

The user asked whether the default queries could be unified with Sent. The
answer runs the other way: Sent joins the saved queries rather than the saved
queries becoming hardcoded.

Inbox, Unread and Important are complete strings that depend on nothing and can
never go stale, so generating them would buy nothing and would cost the four
things the file just gained: reordering, unpinning, renaming and deleting.
Hardcoding them would also make them undeletable, which is a regression for
anyone who does not want one of them.

Sent is different only in that its query CANNOT be stored: it is composed from
every account's `sent` key, so a stored copy goes stale the moment a folder is
renamed. That is a property of Sent, not of "default queries". Storing the
GENERATOR rather than its output keeps both halves: `"generated": "sent"` still
resolves from the accounts at click time, and the entry is an ordinary row that
can be reordered, renamed, unpinned or removed. The row now follows one rule
instead of carrying one member the user did not own.

Two properties had to travel with the entry. The composed query, resolved
through Config::resolvedQuery() so what lands in the bar is what actually ran;
and FLAT mode, since a sent view lists messages and a threaded one folds every
reply back into the conversation the user sent one message into. The sent
generator implies flat rather than trusting the file to say so, because a
hand-edited row would otherwise produce a threaded sent view.

An unknown generator is reported but the row is KEPT: a later build may know it,
and dropping it here would delete it from the file on the next save, which is
the same data loss the unknown-field handling exists to prevent. A generator
whose accounts configure nothing is skipped entirely, exactly as the hardcoded
button was hidden rather than offering one that finds nothing.

Eight new tests. The four pre-existing Sent tests reach this through migration
and were left alone, which is what proves the migrated path still behaves; the
new ones cover a STORED file, which is the path every launch after the first
takes. Mutations: a generator resolving to nothing fails three, ignoring flat
fails two, and not skipping an empty generator fails one. A rename test guards
the property the change exists for, since anything keyed on the literal name
"Sent" would break it.

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

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