Converting and Stabilizing output voltage of a dynamo hub
Don't use a bridge rectifier.  It's inefficient, because you need two diode drops in series,
and it doesn't allow you to use the frame as ground for a DC lamp (like, a modern LED bike light).  Instead, consider a voltage doubler







simulate this circuit &ndash; Schematic created using CircuitLab

The 'regulator' can be a current regulator, or just a Zener followed by a limit resistor.  Several LEDs in series can make use of the 'extra' voltage supplied.
Accuracy, like efficiency, is less than important in this application.

I've used a 6V sealed lead-acid battery as the 'regulator'; overcharging
seems not to be a problem, and a low-dropout regulator  IC can ensure
that the output to the lamps is a stable 6V.

Charging from a '5V' source is possible, but depending on such things
is less than satisfying; the generator has all the power you need.