test(sync): cover FullSync's phase transitions end-to-end

TestFillPendingDetails_ReportsPhaseAwareProgressThroughActivitiesAndWorkoutsPhases
only ever called FillPendingDetails directly, never exercising
FullSync's own setProgress(PhaseDiscovering, ...) / deferred
setProgress(PhaseIdle, ...) wiring around backfillCore/
incrementalSyncCore -- what handleSyncRun actually runs in production
and what SyncModal's "Discovering activities..." spinner depends on.

Add a mock.Client.Delay field (slept, ctx-cancellable, at the start of
GetActivities) so a test can give the discovering phase real
wall-clock duration, then add
TestFullSync_ReportsPhaseTransitionsThroughDiscoveringToIdle, which
runs FullSync in a goroutine and polls Progress() to confirm it
observes PhaseDiscovering mid-flight and PhaseIdle after completion.

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

backend/internal/sync/service_test.go

@@ -630,6 +630,81 @@ func TestFullSync_RecordsOneCombinedSyncRun(t *testing.T) {
 	}
 }
 
+// waitForPhase polls svc.Progress() until it reports phase or timeout
+// elapses, returning whether it was observed. Polling (rather than a single
+// fixed-delay sample) keeps this robust against slower/loaded CI machines.
+func waitForPhase(svc *Service, phase string, timeout time.Duration) bool {
+	deadline := time.Now().Add(timeout)
+	for {
+		if svc.Progress().Phase == phase {
+			return true
+		}
+		if time.Now().After(deadline) {
+			return svc.Progress().Phase == phase
+		}
+		time.Sleep(2 * time.Millisecond)
+	}
+}
+
+// TestFullSync_ReportsPhaseTransitionsThroughDiscoveringToIdle exercises
+// FullSync itself (the actual production call path via handleSyncRun),
+// unlike TestFillPendingDetails_ReportsPhaseAwareProgressThroughActivitiesAndWorkoutsPhases
+// below, which only calls FillPendingDetails directly and so never touches
+// FullSync's own s.setProgress(PhaseDiscovering, ...) / deferred
+// s.setProgress(PhaseIdle, ...) wiring around backfillCore/
+// incrementalSyncCore -- what the frontend's SyncModal "Discovering
+// activities..." spinner actually depends on.
+func TestFullSync_ReportsPhaseTransitionsThroughDiscoveringToIdle(t *testing.T) {
+	db := openTestDB(t)
+	ctx := context.Background()
+	userID := provisionTestUser(t, db)
+
+	const delay = 200 * time.Millisecond
+	workoutID := int64(101)
+	m := &mock.Client{
+		Delay: delay, // gives backfillCore/incrementalSyncCore's GetActivities calls real wall-clock duration
+		Activities: []garmin.Activity{
+			{ActivityID: 1, ActivityType: garmin.ActivityType{TypeKey: "running"}, WorkoutID: &workoutID,
+				StartTimeGMT: "2026-07-05 06:00:00", Distance: 5000, Duration: 1500},
+		},
+		Splits:   map[int64]garmin.ActivitySplits{1: {ActivityID: 1}},
+		Details:  map[int64]garmin.ActivityDetails{1: {ActivityID: 1}},
+		Workouts: map[int64]garmin.Workout{workoutID: {}},
+	}
+	svc := NewService(m, db, userID, Config{BackfillWindowDays: 10},
+		fixedNow(time.Date(2026, 7, 11, 0, 0, 0, 0, time.UTC)))
+	setBackfillHorizon(t, db, userID, 10)
+
+	if p := svc.Progress(); p.Phase != PhaseIdle {
+		t.Fatalf("Progress before FullSync = %+v, want PhaseIdle", p)
+	}
+
+	done := make(chan error, 1)
+	go func() { done <- svc.FullSync(ctx, 10) }()
+
+	// FullSync sets PhaseDiscovering synchronously before backfillCore even
+	// runs; the mock's Delay on each of backfillCore's and
+	// incrementalSyncCore's one GetActivities call (~2*delay total) gives
+	// this comfortable real wall-clock time to observe it before the run
+	// reaches FillPendingDetails/Idle.
+	if !waitForPhase(svc, PhaseDiscovering, delay) {
+		t.Fatalf("Progress().Phase never observed as %q while backfillCore/incrementalSyncCore were running; got %+v", PhaseDiscovering, svc.Progress())
+	}
+
+	select {
+	case err := <-done:
+		if err != nil {
+			t.Fatalf("FullSync: %v", err)
+		}
+	case <-time.After(5 * time.Second):
+		t.Fatal("FullSync did not complete in time")
+	}
+
+	if final := svc.Progress(); final.Phase != PhaseIdle || final.Done != 0 || final.Total != 0 {
+		t.Errorf("Progress after FullSync returns = %+v, want zero-value PhaseIdle", final)
+	}
+}
+
 func TestFillPendingDetails_ReportsPhaseAwareProgressThroughActivitiesAndWorkoutsPhases(t *testing.T) {
 	db := openTestDB(t)
 	ctx := context.Background()