Show a QR code for device-code login (text dialog fallback)

[magne4000]

Summary

The device-code login now shows a scannable QR code of the Twitch activation URL alongside the short user code, so viewers can sign in with their phone camera instead of typing a URL. Both the regular and the private (Turbo/ad-free) login get it, since they share do_device_login().

How it works

• QR is rendered dependency-free: a vendored QR generator (Project Nayuki, MIT) produces the module matrix and a small zlib-based PNG writer emits the image — no PIL/Pillow or other binary library required.
• Shown in a lightweight WindowDialog that polls for the token on a background thread.
• Fallback: any problem with the QR path (render failure, an image-less skin, an exception building the window) falls back to the existing text-only DialogProgress flow, which is preserved unchanged, so login still works everywhere. Polling, slow_down, cancel, expiry and Kodi-shutdown handling are shared by both paths.

Changes

• routes/device_login.py — QR window + shared poller, text flow kept as fallback.
• addon/qr.py (new) — QR→PNG rendering.
• addon/vendor/qrcodegen.py (new, vendored, MIT) + addon/vendor/__init__.py.
• New strings #30334–#30336.

Backward compatibility

Additive; no settings or stored-token changes. Falls back to the current text flow on any device where the QR window can't be shown.

New strings

• #30334 "Scan the QR code, or go to [B]%s[/B]"
• #30335 "Enter this code: [B]%s[/B]"
• #30336 "Waiting for authorization... (press Back to cancel)"

Testing

• QR window shows; scanning it completes sign-in (regular login)
• Same for the private (Turbo) login
• Back/cancel and code-expiry behave correctly
• Text fallback still works (e.g. force a QR-render failure)

Note: the vendored qrcodegen.py accounts for most of the diff size.
Independent of my other two PRs.