i18n: translate the trash strings, and document the trash key

The three new strings from the cleanup action, translated into Italian.
lrelease reports 383 finished and 0 unfinished; an unfinished string is
silently dropped and ships as English inside an otherwise Italian UI.

The changelog gains an Upgrading section for the mandatory `trash` key, the new
optional `inbox` key and the `Del` binding, and states the consequence that
cost real mail on this branch: a folder name that does not match the server is
created rather than reported, mbsync adopts it, and under Create Both it
propagates to the server where other clients see it.

CLAUDE.md is corrected on two counts. Adding an action is five places, not
four; the fifth is a menu, and nothing enforced it until this branch added
everyActionIsReachableFromAMenu(). And the trash design is recorded: why the
origin lives in a tag, why those tags are joined by a tab rather than a space,
and why Restore resolves against the database rather than the model.

Also repairs a race in deletingTwiceLeavesNoOriginTagBehind(). Its guard ran a
query through the bar in the gap between the file rename and the tag writes,
and a query bar run in that gap returns zero rows forever, since QTRY_VERIFY
re-reads rowCount() and never re-runs the query. Measured 3 failures in 12
runs, each burning a full 15s timeout; 0 in 8 after asking the database
directly, with the runtime down from 45s to 0.3s.

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

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