fix: date-based memories are misclassified as location memories

What happened?
Date-only memories are created with `center_lat = 0` and `center_lon = 0` in the backend, but the frontend checks `center_lat == null || center_lon == null` to determine whether a memory is date-based.

As a result, date-based memories can be treated as location memories.

Why this matters
- incorrect badge shown on memory cards
- incorrect title formatting for date-based memories
- incorrect location/date counts and filtering in the Memories page

Relevant code
- backend/app/utils/memory_clustering.py
- frontend/src/components/Memories/MemoryCard.tsx
- frontend/src/components/Memories/MemoryDetail.tsx
- frontend/src/components/Memories/MemoriesPage.tsx

Expected behavior
Date-based memories should be consistently identified either by:
- returning `null` for `center_lat` / `center_lon` from the backend, or
- using the explicit memory `type` field in the frontend instead of inferring from coordinates.