feat(activity-calendar): aggregate by ViewContext.timeBasis

Fixes the inconsistency where switching the memo list to update_time
left the activity heatmap aggregating by created_time. The heatmap
now follows the same time basis as the list it sits next to.

Backend
- UserStats gains memo_updated_timestamps (additive proto field, tag 8).
- GetUserStats and ListAllUserStats populate it alongside the existing
  memo_created_timestamps. No DB migration; memo.updated_ts already
  exists on every row.

Frontend
- useFilteredMemoStats reads timeBasis from ViewContext and selects
  the matching timestamp source.
- StatisticsView and MonthNavigator forward timeBasis through to
  MonthCalendar / YearCalendar so tooltip text matches the basis
  ("X memos in DATE" vs "X memos updated on DATE").
- Falls back to memoCreatedTimestamps when an old server returns an
  empty memoUpdatedTimestamps array (detected by length divergence,
  since protobuf-es deserializes missing repeated fields as []).

Tests
- Backend: TestGetUserStats_MemoUpdatedTimestamps verifies the field
  is populated and reflects post-creation updates.
- Frontend: filtered-memo-stats covers create/update source switching
  and the old-server fallback path; activity-calendar-tooltip covers
  basis-aware label selection.

Spec and implementation plan committed under docs/superpowers/.

server/router/api/v1/test/user_service_stats_test.go

@@ -110,3 +110,51 @@ func TestGetUserStats_TagCount(t *testing.T) {
 	require.Error(t, err)
 	require.Contains(t, err.Error(), "user not found")
 }
+
+func TestGetUserStats_MemoUpdatedTimestamps(t *testing.T) {
+	ctx := context.Background()
+
+	ts := NewTestService(t)
+	defer ts.Cleanup()
+
+	user, err := ts.CreateHostUser(ctx, "ts-user")
+	require.NoError(t, err)
+	userCtx := ts.CreateUserContext(ctx, user.ID)
+
+	memo, err := ts.Store.CreateMemo(ctx, &store.Memo{
+		UID:        "ts-memo-1",
+		CreatorID:  user.ID,
+		Content:    "first content",
+		Visibility: store.Public,
+	})
+	require.NoError(t, err)
+	require.NotNil(t, memo)
+
+	// SQLite UpdateMemo only sets fields explicitly passed (created_ts default
+	// fires on INSERT only). So bump updated_ts explicitly to simulate an edit
+	// happening after creation.
+	newContent := "second content"
+	newUpdatedTs := memo.UpdatedTs + 100
+	require.NoError(t, ts.Store.UpdateMemo(ctx, &store.UpdateMemo{
+		ID:        memo.ID,
+		Content:   &newContent,
+		UpdatedTs: &newUpdatedTs,
+	}))
+
+	userName := fmt.Sprintf("users/%s", user.Username)
+	resp, err := ts.Service.GetUserStats(userCtx, &v1pb.GetUserStatsRequest{Name: userName})
+	require.NoError(t, err)
+	require.NotNil(t, resp)
+
+	require.Len(t, resp.MemoCreatedTimestamps, 1, "should have one created timestamp")
+	require.Len(t, resp.MemoUpdatedTimestamps, 1, "should have one updated timestamp")
+
+	require.Equal(t, memo.CreatedTs, resp.MemoCreatedTimestamps[0].AsTime().Unix())
+	require.Equal(t, newUpdatedTs, resp.MemoUpdatedTimestamps[0].AsTime().Unix())
+	require.Greater(
+		t,
+		resp.MemoUpdatedTimestamps[0].AsTime().Unix(),
+		resp.MemoCreatedTimestamps[0].AsTime().Unix(),
+		"updated_ts should be after created_ts after an edit",
+	)
+}

server/router/api/v1/user_service_stats.go

@@ -100,6 +100,7 @@ func (s *APIV1Service) ListAllUserStats(ctx context.Context, _ *v1pb.ListAllUser
 					Name:                  "",
 					TagCount:              make(map[string]int32),
 					MemoCreatedTimestamps: []*timestamppb.Timestamp{},
+					MemoUpdatedTimestamps: []*timestamppb.Timestamp{},
 					PinnedMemos:           []string{},
 					MemoTypeStats: &v1pb.UserStats_MemoTypeStats{
 						LinkCount: 0,
@@ -113,6 +114,7 @@ func (s *APIV1Service) ListAllUserStats(ctx context.Context, _ *v1pb.ListAllUser
 			stats := userMemoStatMap[memo.CreatorID]
 
 			stats.MemoCreatedTimestamps = append(stats.MemoCreatedTimestamps, timestamppb.New(time.Unix(memo.CreatedTs, 0)))
+			stats.MemoUpdatedTimestamps = append(stats.MemoUpdatedTimestamps, timestamppb.New(time.Unix(memo.UpdatedTs, 0)))
 
 			// Count memo stats
 			stats.TotalMemoCount++
@@ -205,6 +207,7 @@ func (s *APIV1Service) GetUserStats(ctx context.Context, request *v1pb.GetUserSt
 	}
 
 	createdTimestamps := []*timestamppb.Timestamp{}
+	updatedTimestamps := []*timestamppb.Timestamp{}
 	tagCount := make(map[string]int32)
 	linkCount := int32(0)
 	codeCount := int32(0)
@@ -231,6 +234,7 @@ func (s *APIV1Service) GetUserStats(ctx context.Context, request *v1pb.GetUserSt
 
 		for _, memo := range memos {
 			createdTimestamps = append(createdTimestamps, timestamppb.New(time.Unix(memo.CreatedTs, 0)))
+			updatedTimestamps = append(updatedTimestamps, timestamppb.New(time.Unix(memo.UpdatedTs, 0)))
 			// Count different memo types based on content.
 			if memo.Payload != nil {
 				for _, tag := range memo.Payload.Tags {
@@ -262,6 +266,7 @@ func (s *APIV1Service) GetUserStats(ctx context.Context, request *v1pb.GetUserSt
 	userStats := &v1pb.UserStats{
 		Name:                  fmt.Sprintf("%s/stats", BuildUserName(user.Username)),
 		MemoCreatedTimestamps: createdTimestamps,
+		MemoUpdatedTimestamps: updatedTimestamps,
 		TagCount:              tagCount,
 		PinnedMemos:           pinnedMemos,
 		TotalMemoCount:        totalMemoCount,