refactor: migrate pubky images to coil

[ovitrif]

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 SubcomposeAsyncImage with loading/error slots
• 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

Preview

No visual changes — same loading spinner, circular-clipped image, and error fallback.

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)

🤖 Generated with Claude Code

[jvsena42]

Very nice refactor!

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

[ovitrif]

Very nice refactor!

Thank you!
I still have to go over why Claude picked SubcomposeAsyncImage and let it convince me, the coil docs say it's less performant than the more-common AsyncImage.

Drafting the PR to avoid accidental merges until that's cleared.