chore: add Compose Preview guideline to CLAUDE.md

https://claude.ai/code/session_0115Vp2KXGzZC7XCFt93Wu2F

Author: claude

CLAUDE.md

@@ -77,6 +77,7 @@ Example: `#2025 feat: add button to report bug on home screen`
 - Check `KeyMapperIcons` before using other icon sources
 - Use `LocalUriHandler.openUriSafe` extension for URL launching — do not hoist URL launching logic up the call stack
 - Use import statements; never use fully qualified names in Compose code
+- Write `@Preview` composables for every screen
 
 ## Adding a New Action (10-step checklist)