Add system font size as a device attribute

[claude]

Requested by Jake Mor · Slack thread

Before

There was no device attribute exposing the user's system font size, so paywalls couldn't adapt to the device's text-size accessibility setting.

After

Adds two device attributes describing the system font size:

• fontScale: the raw system text-scale multiplier as a Float. The system default is 1.0; larger accessibility text scales report proportionally larger values (e.g. 1.3).
• fontSize: a normalized integer in px derived from the same scale, using a 16px reference base so that the system default (scale 1.0) reports 16. Larger scales report proportionally larger values (e.g. scale 1.3 → 21).

Android has no OS-level content-size category, so no category attribute is reported.

How

• DeviceHelper.fontScale returns context.resources.configuration.fontScale.
• DeviceHelper.fontSize computes (context.resources.configuration.fontScale * 16).roundToInt().
• DeviceTemplate gains fontScale: Float and fontSize: Int fields (JSON keys fontScale and fontSize), populated in getTemplateDevice().
• DeviceTemplateTest and the CHANGELOG are updated accordingly.