[FOEPD-3392] Implement image embedding caching in IndexedDB

[yrahal]

What changes are proposed in this pull request?

Caches SAM2 encoder embeddings in IndexedDB with in-memory LRU eviction so repeated prompts on the same image skip encoding and run decoder only (~150ms vs ~1-3s)

How is this patch tested? If it is not, please explain why.

• Unit tests
• In-app (local uncommitted UI changes for testing)

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What areas of FiftyOne does this PR affect?

• App: FiftyOne application changes
• Build: Build and test infrastructure changes
• Core: Core fiftyone Python library changes
• Documentation: FiftyOne documentation changes
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