Florin,
The problem is that you, like the vast majority of people, have confused "group velocity" for "phase velocity". Einstein simply DEFINED the phase velocity of light, in a vacuum, to be a constant. He said nothing about group velocity being constant.
Group velocity is the velocity at which the envelope of a signal propagates, such as might be measured by timing the arrival of a leading edge of a pulse.
Phase velocity, on the other hand, is the velocity of a signal with a constant envelop - in other words, an infinitely long signal that has no leading or trailing edge. Since the envelop is constant, no measurement of it will provide any indication of a non-zero velocity, whatsoever.
So how do you then DIRECTLY measure the velocity of an infinitely long, ideal sinusoid, that never interacts with anything, (because it is in a vacuum)? The answer is that you can't. You can only measure a changing phase. But that could be caused by EITHER a non-zero velocity, or a change in its frequency, or any combination of the two. So which is it? There is no way to tell. Not even in principle. So just pick one or the other as being CONSTANT, and attribute the phase shift has being entirely caused by the other.
Einstein simply observed that the math transformation would be simpler, if you picked the velocity to be constant, rather than the frequency. Hence, we have Doppler frequency shifts and constant phase velocity, rather than constant frequency and Doppler velocity shifts.
Rob McEachern