feat: Add Compose Multiplatform Desktop support foundation

- Add desktopApp module with JVM entry point and sidebar navigation
- Add Claude specs for AI-assisted development:
  - Agent definitions: nostr-protocol, kotlin-multiplatform, compose-ui, kotlin-coroutines
  - Skills: quartz-kmp conversion, compose-desktop patterns
  - Commands: desktop-run, nip, extract
- Update Gradle configuration with Compose Multiplatform 1.7.1 plugin
- Add coroutines and secp256k1 JVM dependencies to version catalog

Next steps:
- Convert Quartz library to full KMP (expect/actual for crypto)
- Implement relay connections in desktop app
- Share UI components between Android and Desktop

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.claude/commands/extract.md

@@ -0,0 +1,50 @@
+---
+description: Extract a composable from amethyst to shared code
+---
+
+Extract the component `$ARGUMENTS` from the Android app to shared KMP code:
+
+## Process
+
+1. **Locate the component** in the amethyst module:
+   ```bash
+   find amethyst/src -name "*$ARGUMENTS*" -o -name "*$ARGUMENTS*"
+   grep -r "fun $ARGUMENTS\|class $ARGUMENTS" amethyst/src/
+   ```
+
+2. **Analyze dependencies**:
+   - Android-specific imports (Context, Intent, etc.)
+   - Platform APIs (Camera, MediaStore, etc.)
+   - Android Compose specifics vs standard Compose
+
+3. **Identify what can be shared**:
+   - Pure Composable functions → `shared-ui/commonMain/`
+   - Business logic → `quartz/commonMain/`
+   - Platform-specific → create expect/actual
+
+4. **Create shared version**:
+   - Move to appropriate shared module
+   - Replace Android imports with multiplatform alternatives
+   - Add expect declarations for platform-specific parts
+
+5. **Update references**:
+   - Change imports in amethyst module
+   - Add implementations in desktopApp if needed
+
+## Common Replacements
+
+| Android | Multiplatform |
+|---------|---------------|
+| `LocalContext.current` | expect/actual or parameter |
+| `stringResource()` | `Res.string.*` |
+| `painterResource()` | `painterResource(Res.drawable.*)` |
+| `Toast.makeText()` | Custom snackbar/notification |
+| `Intent` | expect/actual for navigation |
+
+## Example
+
+```
+/extract NoteCard
+```
+
+This will find NoteCard, analyze its dependencies, and guide you through extracting it to shared code.