feat: Implement multi-voice support and management

Refactor the TTS handling to support multiple, user-selectable voices. This replaces the previous single-voice system.

Key changes:
- Introduce VoiceManager to handle loading and managing voices from a dedicated oices/ directory.
- Add slash commands (/voice list, /set, /current, /refresh) for users to manage their personal TTS voice.
- Implement on-demand voice loading to improve startup time and memory usage.
- Remove the old 	ts_handler.py and single voice .wav files in favor of the new system.
- Update configuration to specify a voices directory instead of a single file path.

config.py

@@ -7,7 +7,8 @@ load_dotenv()
 class Config:
     DISCORD_TOKEN: str = os.getenv("DISCORD_TOKEN", "")
     TEXT_CHANNEL_ID: int = int(os.getenv("TEXT_CHANNEL_ID", "0"))
-    VOICE_WAV_PATH: str = os.getenv("VOICE_WAV_PATH", "./voice.wav")
+    VOICES_DIR: str = os.getenv("VOICES_DIR", "./voices")
+    DEFAULT_VOICE: str | None = os.getenv("DEFAULT_VOICE", None)
 
     @classmethod
     def validate(cls) -> list[str]:
@@ -17,6 +18,6 @@ class Config:
             errors.append("DISCORD_TOKEN is not set")
         if cls.TEXT_CHANNEL_ID == 0:
             errors.append("TEXT_CHANNEL_ID is not set")
-        if not os.path.exists(cls.VOICE_WAV_PATH):
-            errors.append(f"Voice WAV file not found: {cls.VOICE_WAV_PATH}")
+        if not os.path.exists(cls.VOICES_DIR):
+            errors.append(f"Voices directory not found: {cls.VOICES_DIR}")
         return errors