Restore last viewport on launch; skip auto-fly when restored

[dfalling]

Summary

• Persist map center + zoom to EncryptedStorage on each region change (debounced 500ms) via a small viewportStore.
• On launch, hydrate the saved viewport and pass it to Camera.initialViewState so element fetches happen immediately for the restored region — no more zoom-1 world view + fly-to-user round trip.
• When a saved viewport exists, the auto-fly-to-user branch is skipped. The recenter button is the explicit way to jump to your current location.
• First-launch users (no saved viewport) keep the fly-to-user behavior so they don't see a blank world.
• Refactored the off-center check into a useMemo driven by position + last viewportState, so the recenter button can appear as soon as position arrives — even before any pan.

Reviewer notes

• The 4th bullet in the request ("keep the zoom until the first batch of elements is loaded") is currently implemented as "never auto-change zoom when a saved viewport exists." Easy to revert if it feels wrong in practice.
• Storage uses the existing react-native-encrypted-storage for consistency with tokenStore. Bounds aren't sensitive, but adding a second storage dep felt like more churn than it's worth.

Test plan

• First-ever launch (clear app data): map lands at world view, then flies to user when GPS arrives. After a pan, kill + relaunch: lands at last-panned viewport, no fly.
• Panned away from user: recenter button appears; tapping it flies to user.
• Open app while physically far from last saved viewport: recenter button appears as soon as GPS resolves.
• Element pins fetch and render immediately on launch for the restored region, no flicker through world view.

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