[fix][meetup] Give each group its own timezone in a multi-group run

[duxi90]

Description

• groupTimezones (keyed by group urlname, like groupHosts) puts the zone on
  GroupTarget, so createMeetupDrafts converts per group instead of once. One run now
  renders 16:00Z as 11:00 Chicago / 18:00 Belgrade / 12:00 Toronto — one instant, three
  wall clocks.
• New list-groups command prints each configured group's name and the timezone Meetup
  holds for it, plus a paste-ready groupTimezones map, mirroring list-venues. A group
  that cannot be read is warned about rather than aborting the listing.
• A partial multi-group failure now actually exits 1. process.exitCode = 1 was being
  overwritten by runMain(...).then(() => process.exit(0)), so those runs exited 0.

BREAKING: a multi-group run carrying only the shared timezone now throws, naming the
groups that need an entry. One wall time has no correct interpretation across several
zones, and failing loudly beats scheduling a talk at the wrong hour in front of an
audience. Single-group configs and narrowed runs (--groups cpp-serbia) still accept
timezone unchanged. This lands after 0.3.0, so it needs a 0.4.0 bump before the next tag.

Motivation

Cross-posting one session to a Pro network put it at a different instant in every group —
a talk at 16:00Z was created as 11:00 in Chicago (right), 11:00 in Belgrade (7h early) and
11:00 in Toronto (1h early) — because Meetup reads startDateTime as wall time in the
receiving group's own zone and accepts no offset.

Testing

Tested locally. Full CI chain green (build, lint, format:check, typecheck, spell, test),
125 tests — 11 new, covering per-group conversion, case-insensitive groupTimezones keys,
the multi-group refusal and the groups it names, the single-group and narrowed-run
exemptions, and both multi-group exit codes through the real bin. list-groups is untested
beyond typecheck: it needs live credentials. It is read-only, one groupByUrlname query
per group.

Checkup list

• Labeled
• Assigned to a project
• Linked issue(s) (to close or track progress)
• Documentation updated
• Test coverage