feat: implement owner check for slash_change_avatar command and improve error handling

cogs/bot_appearance_cog.py

@@ -4,6 +4,11 @@ from discord import app_commands
 import httpx
 import io
 
+# --- Helper: Owner Check ---
+async def is_owner_check(interaction: discord.Interaction) -> bool:
+    """Checks if the interacting user is the bot owner."""
+    return interaction.user.id == interaction.client.owner_id
+
 class BotAppearanceCog(commands.Cog):
     def __init__(self, bot):
         self.bot = bot
@@ -63,7 +68,7 @@ class BotAppearanceCog(commands.Cog):
         image_url="A direct URL to the image for the new avatar (optional if attachment is provided).",
         attachment="An image file to use as the new avatar (optional if URL is provided)."
     )
-    @app_commands.checks.is_owner()
+    @app_commands.check(is_owner_check)
     async def slash_change_avatar(self, interaction: discord.Interaction, image_url: str = None, attachment: discord.Attachment = None):
         """Changes the bot's global avatar. Accepts a direct image URL or an attachment."""
         await interaction.response.defer(ephemeral=True)
@@ -127,7 +132,7 @@ class BotAppearanceCog(commands.Cog):
     async def cog_app_command_error(self, interaction: discord.Interaction, error: app_commands.AppCommandError):
         if isinstance(error, app_commands.MissingPermissions):
             await interaction.response.send_message("You don't have the required permissions (Administrator) to use this command.", ephemeral=True)
-        elif isinstance(error, app_commands.NotOwner):
+        elif isinstance(error, app_commands.CheckFailure):
             await interaction.response.send_message("This command can only be used by the bot owner. If you wish to customize your bot's appearance, please set up a custom bot on the web dashboard.", ephemeral=True)
         else:
             print(f"Error in BotAppearanceCog (app_command): {error}") # Log other errors to console