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/.