What portion of the electromagnetic spectrum do cats see?
Short answer
Spectral sensitivity of cats indeed ventures into the UV, but not beyond ~320 nm. Their maximum is likely similar to ours, i.e., ~750 nm.

Background
The spectral sensitivity of blue cones (photoreceptors detecting low wavelengths) of many species, including humans and cats, extends into the UV range (Fig. 1).


Fig. 1. Human cone absorption spectra. Source: University of Kentucky

However, the various structures in the eye, most notably the lens, filter out UV in humans (Fig. 2). Indeed, surgical removal of the lens in humans results in enhanced sensitivity to UV light below 420 nm (Griswold &amp; Stark, 1992). 


Fig. 2. Human transmission spectra. Source: Columbia University

In the cat, however, the lens transmits a large portion of the UV light in the range of 320 - 400 nm, while primates transmit virtually no light of these wavelengths. Figure 3 shows the lens transmission spectra of various species. At 50% transmission the second (dotted) line from the left is the cat (50% transmission at ~340 nm), while a primate (squirrel monkey, species representative for humans and other primates) in this graph is line number 7 from the left with (50% transmission at ~410 nm). Hence, cat lenses absorb less light in the UV region, but transmission is virtually nil at 320 nm (Fig. 3). Given that their cones, just like humans, are sensitive to UV, we can reasonably expect that cats can see in the UV region including wavelengths of 320 nm and up. Cats also have the 560 nm (red) cones, like we do, so their maximum wavelength is expectedly similar to ours (~750 nm).  


Fig. 3. Lens transmission spectra of various species. Source: Douglas &amp; Jeffery, 2014   

References
- Douglas &amp; Jeffery, Proc R Soc B (2014); 281: 20132995
- Griswold &amp; Stark, Vis Res (1992); 32(9): 1739-43