fix: resolve addon conflicts, Classic Era support, and sound playback

- Fix "Addon already exists" error with safe addon creation
- Fix sounds not playing (corrected TOC load order)
- Fix /blu command in Classic Era (multi-version options panel)
- Consolidate initialization.lua into core.lua
- Add FEATURE_AVAILABILITY table for version-aware features
- Add BLU_Cata.toc for Cataclysm Classic
- Update README with Supported Events Matrix
- Updated .gitignore
- Updated Discord webhook
- Fix version number in options panel