feat: add /voice preview command

- Added 8 random preview sample lines for voice testing
- New /voice preview <name> command to hear voices before selecting
- Previews play in queue like regular messages (no queue jumping)
- Preview does NOT change user's active voice preference
- Updated queue system to support voice override for previews
- Added documentation for new command in README

README.md

@@ -109,6 +109,7 @@ A Discord bot that reads messages aloud using [Pocket TTS](https://github.com/ky
    - `/voice set <name>` - Change your personal TTS voice
    - `/voice current` - Shows your current voice
    - `/voice refresh` - Re-scan for new voice files (no restart needed)
+   - `/voice preview <name>` - Preview a voice before selecting it
 
 ### Test Mode