Fix QR codes rendered wide/horizontal on ILI9341 displays

[Copilot]

On ILI9341 displays (240×320 physical), Renderer swaps canvas dimensions to canvas_width=320, canvas_height=240 so the 90° hardware rotation in show_image produces a correct portrait layout. QRDisplayScreen was blindly passing those non-square dimensions to the QR encoder, which stretched the square QR image to 320×240 — producing the wide/horizontal QR codes seen in B10.

Fix: compute qr_size = min(canvas_width, canvas_height) once before the render loop and use it for both axes.

# before
image = self.qr_encoder.next_part_image(
    self.renderer.canvas_width, self.renderer.canvas_height, ...)

# after
qr_size = min(self.renderer.canvas_width, self.renderer.canvas_height)
image = self.qr_encoder.next_part_image(qr_size, qr_size, ...)

No behavioural change on square (240×240 ST7789) displays.

This pull request is categorized as a:

• New feature
• Bug fix
• Code refactor
• Documentation
• Other

Checklist

• I've run pytest and made sure all unit tests pass before sumbitting the PR

If you modified or added functionality/workflow, did you add new unit tests?

• No, I'm a fool
• Yes
• N/A

I have tested this PR on the following platforms/os:

• Raspberry Pi OS Manual Build
• SeedSigner OS on a Pi0/Pi0W board
• Other

Note: Keep your changes limited in scope; if you uncover other issues or improvements along the way, ideally submit those as a separate PR. The more complicated the PR the harder to review, test, and merge.