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- Removes compose-desktop.md (superseded by desktop-expert/).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/desktop-expert/references/keyboard-shortcuts.md

@@ -365,9 +365,9 @@ fun testOsDetection() {
 
 ---
 
-## Current Issues in Amethyst
+## Current Pattern in Amethyst
 
-**Main.kt:105-123** hardcodes `ctrl = true`:
+`Main.kt` (MenuBar starting at L234) already branches on `isMacOS` (L120) for every shortcut. When adding a new menu item, follow the same pattern — **do not** hardcode `ctrl = true`:
 
 ```kotlin
 // ❌ WRONG: Hardcoded Ctrl (doesn't work on macOS)
@@ -378,7 +378,7 @@ Item(
 )
 ```
 
-**Fix:**
+**Current pattern (what Main.kt does):**
 
 ```kotlin
 // ✅ CORRECT: OS-aware