Refreshes .claude/ skill library: fixes stale refs, adds 4 new skills

- Updates CLAUDE.md tech stack to current versions (Compose 1.10.3, Kotlin 2.3.20).
- Reframes kotlin-multiplatform iOS as mature; adds secp256k1-kmp 0.23.0 references.
- Updates desktop-expert Main.kt references (code grew from ~270 to 1341 lines and
  NavigationRail moved to ui/deck/SinglePaneLayout.kt); replaces obsolete
  "hardcoded ctrl = true" anti-pattern note with accurate isMacOS branching.
- Removes compose-desktop.md (superseded by desktop-expert/).
- Adds nostr-expert references: nip19-bech32, event-factory, crypto-and-encryption,
  large-cache. Adds kotlin-expert/common-utilities, compose-expert/rich-text-parsing,
  android-expert/image-loading.
- New skills: account-state (Account + LocalCache), relay-client (subscriptions,
  filter assemblers, preloaders), feed-patterns (FeedFilter + FeedViewModel family),
  auth-signers (NostrSigner across internal / NIP-46 / NIP-55).

.claude/skills/kotlin-expert/SKILL.md

@@ -795,6 +795,7 @@ Passing lambda to function?
 - `references/sealed-class-catalog.md` - All sealed types in quartz
 - `references/dsl-builder-examples.md` - TagArrayBuilder, other DSL patterns
 - `references/immutability-patterns.md` - @Immutable usage, data classes, collections
+- `references/common-utilities.md` - Canonical helpers: `NumberFormatters`, `TimeUtils`, `Hex`, `PubKeyFormatter`, `CoroutinesExt.launchIO`, `OptimizedJsonMapper`, etc.
 
 ### Codebase Examples
 - AccountManager.kt:36-50 - sealed class AccountState, StateFlow pattern