fix: date-based memories are misclassified as location memories

[ajithhraj]

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.

[github-actions]

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[JITENDRAGAHERWAR03]

Hello 👋
I am interested in working on this issue. I have basic knowledge of Python and I’m eager to learn and contribute. I will try to provide a clean and well-documented solution.
Please assign this issue to me if possible.
Thank you 😊