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/api/api_test.go

@@ -335,6 +335,42 @@ func TestListActivities_ReportsLockedForManualAndRaceAssignments(t *testing.T) {
 	}
 }
 
+func TestSyncReset_DeletesActivitiesAndBackfillEndpointIsGone(t *testing.T) {
+	s, db := newTestServer(t)
+	ctx := newCtx()
+
+	if _, err := db.UpsertActivity(ctx, store.Activity{GarminActivityID: 1, ActivityType: "running", StartTimeUTC: "2026-07-11 06:00:00", RawJSON: "{}"}); err != nil {
+		t.Fatalf("UpsertActivity: %v", err)
+	}
+
+	router := s.Router()
+	rec := doJSON(t, router, http.MethodPost, "/api/sync/reset", nil)
+	if rec.Code != http.StatusAccepted {
+		t.Fatalf("reset status = %d, body = %s", rec.Code, rec.Body.String())
+	}
+
+	deadline := time.Now().Add(2 * time.Second)
+	for {
+		activities, err := db.ListActivities(ctx, store.ActivityFilter{})
+		if err != nil {
+			t.Fatalf("ListActivities: %v", err)
+		}
+		if len(activities) == 0 {
+			break
+		}
+		if time.Now().After(deadline) {
+			t.Fatalf("timed out waiting for reset to delete activities, still have %d", len(activities))
+		}
+		time.Sleep(10 * time.Millisecond)
+	}
+
+	// "Full backfill" no longer exists as an endpoint -- superseded by reset.
+	rec = doJSON(t, router, http.MethodPost, "/api/sync/backfill", nil)
+	if rec.Code != http.StatusNotFound {
+		t.Errorf("/api/sync/backfill status = %d, want 404 (removed)", rec.Code)
+	}
+}
+
 func TestProgression_ReturnsSortedTimeSeries(t *testing.T) {
 	s, db := newTestServer(t)
 	ctx := newCtx()