captouch calib: more transparent UX

we've been out of the loop of this topic for a while, but probably we still calibrate captouch silently once on boot and then never again. the calibration values are usually reasonably stable, so we could just put them in persistent storage so that touching a petal during boot does not generally break the experience.

users in generall are not aware of the current process at all which may lead to issues. we propose that if no stored calibration is found, a splash screen will inform users that a calibration cycle is needed and that they should not touch any petals while calibration is happening. a captouch cycle is then triggered (which a mild delay) by a button press, the results are stored in persistent storage.

for recalibrating manually, captouch demo used to implement it (see #84 (closed) ), we can continue using it, or make it a seperate system app (but we don't really see good reason for that). ideally, missing captouch calibration at boot can just start captouch demo with a special flag or smth so that we don't have to write the same splashes twice.