feat(queries): drop pinning, the menu is every saved query's home

Item 94. The query row is the six built-in filters (Unread, Inbox,
Important, Sent, Drafts, Trash), which compose with the account
dropdown, and every saved query lives in the More queries menu. Nothing
has to decide which of the user's queries get button space, which is the
question item 93 would otherwise have had to answer.

SavedQuery::pinned is gone from the struct, the reader, the writer, the
save dialog's checkbox and the pin/unpin context action.

The stored key is stripped rather than left ignored, at the user's
choice. That has one non-obvious requirement: `pinned` stays named in
loadSavedQueries' `known` list precisely so it is NOT collected as an
unknown field, since those are preserved and written straight back. A
mutation removing that name puts the key in the file for ever.

Confirmed with the user before starting that the built-in set covers
their use, since removing pinning removes the escape hatch this item was
blocked on.

Tests: four pinning tests replaced by two on the new rule, four more
converted from buttons to menu entries. migrationPinsEveryEntry and
aStoredGeneratedQueryIsUnpinnedNotDropped are rewritten around the
property that outlived the flag rather than deleted: an entry must be
KEPT, which is what both assertions were really guarding.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KEcn3u19xPqv6ggD15PG4c

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