voxtype: require the Vulkan backend binary before enabling GPU (#483)

omarchy-hw-vulkan reports whether the machine has a Vulkan runtime, not
whether this voxtype build has a Vulkan backend. The aarch64 package ships
only /usr/lib/voxtype/voxtype-cpu, so on Apple Silicon the runtime check
passes, `voxtype setup gpu --enable` runs, and it fails every time with:

    Error: Vulkan backend not installed.
    The voxtype-vulkan binary was not found in /usr/lib/voxtype

`|| true` keeps the install going, so the result is a working install that
prints a red error in the middle of its own setup terminal. Users reasonably
read that as a failed installation.

Gate on the backend binary as well as the runtime.

Verified on Mac14,10 (M2 Pro, 16-inch 2023), Omarchy 4.0.3rc4-1,
voxtype-bin 1.0.1-1 from omarchy-aarch64:

  omarchy-hw-vulkan                                    -> true
  omarchy-hw-vulkan && [[ -x .../voxtype-vulkan ]]     -> false
  /usr/lib/voxtype/  -> voxtype-cpu, voxtype-onnx, voxtype-osd,
                        voxtype-osd-gtk4, voxtype-osd-quickshell

Dictation still works; it runs on the CPU backend, which is what this
package provides.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>
Co-authored-by: Naeem Malik <malik2015naeem@gmail.com>