Merge backfill into Sync now, replace Full backfill with a destructive Reset all

Sync now now runs Backfill (resumes from the watermark, so a widened history
horizon is picked up automatically) then IncrementalSync then detail-fill, in
one click. "Full backfill" is gone -- in its place, "Reset all" (styled as a
destructive action, gated by a confirm dialog) wipes every synced activity
and its laps/kind assignments and rewinds the backfill watermark, so the next
sync performs a genuinely fresh pull instead of trying to patch up existing
rows with newer schema fields.

backend/internal/sync/service_test.go

@@ -427,6 +427,47 @@ func TestBackfill_SecondRunIsANoOpOnceHorizonFullyCovered(t *testing.T) {
 	}
 }
 
+func TestResetAll_AllowsFreshBackfillAfterwards(t *testing.T) {
+	db := openTestDB(t)
+	ctx := context.Background()
+
+	m := &mock.Client{Activities: []garmin.Activity{
+		{ActivityID: 1, ActivityType: garmin.ActivityType{TypeKey: "running"}, StartTimeGMT: "2026-07-05 06:00:00", Distance: 5000, Duration: 1500},
+	}}
+	svc := NewService(m, db, Config{BackfillHorizonDays: 10, BackfillWindowDays: 10},
+		fixedNow(time.Date(2026, 7, 11, 0, 0, 0, 0, time.UTC)))
+
+	if err := svc.Backfill(ctx); err != nil {
+		t.Fatalf("first Backfill: %v", err)
+	}
+	firstCallCount := m.GetActivitiesCalls
+
+	if err := svc.ResetAll(ctx); err != nil {
+		t.Fatalf("ResetAll: %v", err)
+	}
+	activities, err := db.ListActivities(ctx, store.ActivityFilter{})
+	if err != nil {
+		t.Fatalf("ListActivities: %v", err)
+	}
+	if len(activities) != 0 {
+		t.Fatalf("expected 0 activities after ResetAll, got %d", len(activities))
+	}
+
+	if err := svc.Backfill(ctx); err != nil {
+		t.Fatalf("Backfill after reset: %v", err)
+	}
+	if m.GetActivitiesCalls <= firstCallCount {
+		t.Errorf("expected Backfill after ResetAll to call GetActivities again (fresh pull), call count stayed at %d", m.GetActivitiesCalls)
+	}
+	activities, err = db.ListActivities(ctx, store.ActivityFilter{})
+	if err != nil {
+		t.Fatalf("ListActivities after re-backfill: %v", err)
+	}
+	if len(activities) != 1 {
+		t.Fatalf("expected 1 activity re-fetched after reset+backfill, got %d", len(activities))
+	}
+}
+
 func TestBackfill_ResumesFromWatermarkWhenHorizonGrows(t *testing.T) {
 	db := openTestDB(t)
 	ctx := context.Background()