gmail render: adopt the shared Selection filter from gmail insert

[newhoggy]

Context

#1655 (gmail insert) added src/cli/gmail/selection.rs — a Selection/SelectionArgs pair filtering manifest.jsonl by --all/--since/--until/--id/--ids-from/--source-label. It was built as a shared module specifically so gmail render --archive-dir could adopt it later, per its own doc comment:

Factored out as its own module (rather than inlined into insert.rs) so render can adopt it later — a follow-up, not part of this change.

Problem

gmail render --archive-dir DIR --all (src/cli/gmail/render.rs) currently has exactly one selector: --all, over every non-deleted manifest record. There's no way to render only messages in a date range, or a specific set of ids, without first filtering them into a directory of .eml paths some other way.

Proposal

Replace render's --archive-dir --all pair with #[command(flatten)] selection: SelectionArgs (or a subset of it), reusing Selection::from_args/Selection::matches against manifest.records_not_deleted() the same way gmail insert's engine does. --all should keep working identically (it's already one of Selection's selectors) — this is additive, not a breaking change to the existing invocation.

Non-goals

• No change to render's bare-file-paths mode (omni-dev gmail render *.eml), which has no --archive-dir involvement at all.

Ref #1655.