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>

docs/v3/V3-03-units.md

@@ -1,6 +1,6 @@
 # V3-03 — Distance and speed units
 
-**Phase** Foundations · **Depends on** V3-02 · **Size** S · **Status** Not started
+**Phase** Foundations · **Depends on** V3-02 · **Size** S · **Status** Done
 
 ## Goal
 Imperial as well as metric, chosen once and applied everywhere.
@@ -48,3 +48,40 @@ The obvious trap is converting too deep in the stack. Guard it with the export t
 
 ## Out of scope
 Temperature, pace (min/km) — pace is arguably right for running, revisit after V3-01.
+
+
+## Outcome
+
+Built ahead of V3-02 in execution order, despite the ticket table listing it as
+depending on V3-02 — the Settings screen needed something real to control, and the
+formatting/`Config` plumbing itself has zero dependency on a screen existing. Both are
+done; the numbering is unchanged.
+
+`UnitSystem` lives in `domain/models.dart`, not `ui/format.dart` as first drafted —
+`Config` (a data/preferences-layer class) needed the enum too, and having it depend on
+`ui/` read backwards. Moved to the domain layer alongside `Activity`, which every other
+cross-cutting preference-like enum in this codebase already does.
+
+Reactivity reuses the exact pattern `mapEnabledProvider` already established —
+`unitSystemProvider`, a `StateProvider<UnitSystem>` seeded from `Config` once and then
+read/written directly by the UI — rather than introducing the heavier `ConfigNotifier`
+class the ticket's implementation notes proposed. `Config` mutates its own backing
+`SharedPreferences` in place, so a widget re-assigning the same `Config` instance to
+`configProvider` was never going to notify anything; this sidesteps that without a new
+abstraction.
+
+Threaded through all three screens plus the speed histogram's bucket labels, which
+convert-and-round for display (`formatSpeedRangeLabel`) without changing how
+`speedHistogram` itself bins — binning stays km/h always, matching the invariant that
+storage and computation never see the display unit. Export was the one place explicitly
+*not* touched: `gpx()`/`geoJson()` take no `UnitSystem` parameter at all, which is a
+stronger guarantee than validating one.
+
+One real finding: `config_test.dart`'s locale-default test deliberately does not assert
+a specific value, because it cannot know the test runner's own locale — and that caution
+was immediately vindicated. `settings_screen_test.dart` first asserted a fresh `Config`
+defaults to metric and failed, because this dev machine's own locale resolves to a
+region in the imperial set. Fixed by seeding an explicit value before asserting, the
+same technique already used elsewhere for exactly this reason.
+
+22 tests: 13 `format_test.dart`, 9 `config_test.dart`.