fix(worldclock): fix Etc/GMT names at the fallback, not per caller

The previous rule named every non-place id by its current offset, which
renamed legacy ids such as Japan, called Etc/UTC "GMT" and made a CET
row's name change at DST. Only the Etc/GMT+-n ids have a misleading
last segment, so ZoneCatalog.cityFallbackFor now flips just those to
their real offset. That also reaches the zone picker's device row,
which takes its name from the same fallback.

docs/ARCHITECTURE.md

@@ -2407,7 +2407,7 @@ differ from it, so with no stored home the rule is always null, and Settings ›
 Clock says so under the switch rather than hiding it. A device set by UTC
 offset rather than by city reports an id like `Etc/GMT-9`, whose last segment
 has its sign reversed, so that zone is named by its real offset ("GMT+9")
-instead, on the local row and on the face alike (`ZoneCatalog.hasPlaceName`).
+instead, wherever a zone falls back to its id for a name (`ZoneCatalog.cityFallbackFor`).
 
 The row is not stored. It is `WorldClockUiState.Loaded.local`, built per tick
 by the source from the same `zones.current()` read the face uses, so a