Stacked PR on top of:

• feat: add profile and contacts fetching from pubky #824

Migrates pubky image loading from a manual ViewModel + two-tier cache (PubkyImageCache) to Coil 3, simplifying the architecture and reducing custom code.

Description

• Add Coil 3 dependency (io.coil-kt.coil3:coil-compose:3.2.0)
• Create PubkyImageFetcher — a Coil Fetcher that handles pubky:// URIs via PubkyService.fetchFile(), including JSON file descriptor indirection
• Create ImageModule — Hilt module providing a singleton ImageLoader with memory cache (15% app memory) and disk cache (cacheDir/pubky-images/)
• Replace PubkyImage composable internals: remove PubkyImageViewModel and use Coil's AsyncImage with onSuccess/onError callbacks and a custom ImageOverlay for smooth animated transitions between loading, success, and error states
• Use GradientCircularProgressIndicator for the loading state spinner
• Add spring-pop scale animation on image load success via graphicsLayer
• Remove PubkyImageCache (replaced by Coil's built-in caching)
• Remove PubkyRepo.fetchImage() / cachedImage() (image loading now handled entirely by Coil)
• Update PubkyRepo.signOut() to evict pubky images from Coil caches
• Add unit tests for PubkyImageFetcher (factory filtering, raw data, JSON descriptor following)
• Update PubkyRepoTest to reflect removed image methods
• Update docs/pubky.md to document the new architecture

Note: Uses AsyncImage (standard composition) instead of SubcomposeAsyncImage (subcomposition) — Coil docs recommend this for performance in LazyList contexts like the contacts screen.

Preview

QA Notes

• Verify pubky profile images load correctly on the contacts and profile screens
• Verify images are cached (navigate away and back — should load instantly)
• Verify sign-out clears cached images (sign out, sign in with different account)
• Verify loading spinner (gradient circular) appears briefly before image loads
• Verify error fallback icon appears for broken/unreachable URIs

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