docs: spec query bar completion, require translatable strings

Design for backlog item 17. Completion covers query prefixes, tag values,
date values, path values and mimetype values, each carrying a description
so the bar documents the query language while it is typed.

A new QueryCompleter class owns it, with the cursor-context tokenizer as a
pure function so the parsing rules are testable without a widget or a
database. NotmuchWorker gains an all-tags call, which did not exist.

Addresses for from:/to: are out of scope: libnotmuch exposes no all-addresses
call. Both prefixes still appear so the vocabulary reads complete.

Account maildirs belong to path:, not folder:. Account::scopedQuery builds
path:"<maildir>/**", and folder: is a different matcher in notmuch, against
the folder name rather than the directory path.

Also records the translatable-strings rule in CLAUDE.md. Existing code is
not yet audited against it.

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