Add centralized `is_admin(interaction)` helper that includes the
`isinstance(interaction.user, discord.Member)` guard, preventing
AttributeError in DM contexts.
Use it in `can_edit_player_image()` which previously accessed
`guild_permissions.administrator` directly without the isinstance
guard. Update the corresponding test to mock the user with
`spec=discord.Member` so the isinstance check passes.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>