V3-02 + V3-03: settings screen, and metric/imperial units

Built together because V3-02's settings screen needed something real for
V3-03's units to control -- executed in reverse of the ticket numbering, but
both tickets are independently complete.

V3-03: UnitSystem (metric/imperial) lives in domain/models.dart alongside
Activity, defaulting from Platform.localeName on first launch. Storage stays
SI everywhere -- conversion happens only in ui/format.dart, at the last
possible moment, which is now documented as the file's central invariant.
Threaded through all three screens plus the speed histogram's bucket labels,
which relabel for display without changing how speedHistogram itself bins.
Export was deliberately left untouched: gpx()/geoJson() take no UnitSystem
parameter at all, a stronger guarantee than validating one would be.

V3-02: SettingsScreen reachable from the record screen. Map render toggle
(nothing previously exposed mapEnabledProvider to the user, despite it
existing since T15 -- there was nothing to "move off trip detail" as drafted),
unit selector, upload endpoint with http(s) validation, a copyable device id,
and a licences page via Flutter's built-in showLicensePage. Skipped
package_info_plus (static version string instead) and a privacy-policy link
(none published yet) as disproportionate to an S-sized ticket.

Neither ticket needed the ConfigNotifier the implementation notes proposed:
unitSystemProvider reuses the exact StateProvider-seeded-from-Config pattern
mapEnabledProvider already established, since Config mutates its own backing
SharedPreferences in place and re-assigning the same instance would never
notify a watcher anyway.

A real locale-dependent flake was caught, not just anticipated: a test
asserting a fresh Config defaults to metric failed, because this machine's own
locale resolves to a region in the imperial set. Fixed by seeding an explicit
value before asserting, and config_test.dart's locale test was written from
the start to only prove the fallback path doesn't throw, not to assert which
value it returns.

221 tests (211 -> 221): 13 format_test, 9 config_test, 9 settings_screen_test,
1 confirming the record screen's settings button is genuinely wired. Analyze
clean.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

lib/src/domain/models.dart

@@ -33,6 +33,15 @@ enum TripState { recording, paused, completed }
 /// `docs/v3/`.
 enum Activity { motorcycle, bicycle, scooter, skateboard, running, walking, other }
 
+/// Metric or imperial, chosen once as a whole-app preference.
+///
+/// **Display-only, forever.** Storage stays SI (metres, km/h) everywhere else in the
+/// app — converting at the storage layer would corrupt every recorded ride and break the
+/// cross-language parity harness, which compares raw metric values against the Kotlin
+/// original. See `ui/format.dart`, the only place this enum's value should ever change a
+/// number rather than just relabel one. See V3-03 in `docs/v3/`.
+enum UnitSystem { metric, imperial }
+
 /// One ride, from pressing Start to pressing Stop.
 ///
 /// The aggregate fields are denormalised on purpose. They are accumulated as points