How exactly should I build a smart wall light switch for a smart home
EDIT: As suggested
Danger Will Robinson! Mains voltages (or any voltage above 60V) can be very dangerous. If you are toying around with this and get the sudden urge to lick your creation while in operation, I shall not be held responsible for loss of tongue and/or life. And expand upon that as you like for any and all other body parts.
End of Edit
To expand Samuel's comment into an answer, if you use the normal mechanical wall-switch and make sure no wires get stuck anywhere you solve two possible issues:

You have a nice standard-looking switch
You can use dirty tricks to get your low voltage, because the switch already makes it safe.

What do I mean? Well, Samuel said, if I'm not wrong, do something like this:



simulate this circuit &ndash; Schematic created using CircuitLab
In the modified situation, when it is as drawn, the light is off. When you flip the relay the light turns on. When you then flip the switch, it again interrupts the path.
If you make that relay a Latching Relay, such as these: EE2-5SNU NEC Relays you can change their connection with just a single pulse, which will not take much power if you keep the pulse short.
BEAR IN MIND! These are just an example I thought of, they are low current and not very high voltage (may do for light lamps or such, but no big halogen or high power LED lamps or Fluo's). But just to give you an idea.
You can then use a small non-isolated power supply, such as a dirty capacitor divider (they aren't allowed any more in production stuff, AFAIK) to get your low voltage.
For more insights into power supply decisions and why some are better or worse, depending on the application, this Stack answer is a pretty decent one:
https://electronics.stackexchange.com/a/41944/53769
((
With the addition that a viable solution could be to take apart one of those tiny block 5V USB Chargers and making them safe-enough for inside your wall: i.e. glueing everything securely in place to make sure no shorts exist. The cheap eBay ones won't be safe to use for normal stuff, but in the wall, behind a mains-rated switch and rebuilt so that no fire can erupt from a short, they are actually nice, cheap solutions to 5V problems.
))
Because you use a Mains-Rated AC wall switch, you are allowed to power your electronics with any non-isolated solution, because the switch and the wireless module will take care of protecting you. The MCU, Relay and Zigbee module do not care if they are &quot;unsafely connected&quot;, because that only relates to you getting zapped or not.
Just during testing/experimenting you would need to be extra careful not to zap yourself. If you are worried that might be one step too far, you might want to use a regulated fully isolated adapter until you're almost done tinkering. Just for your own safety.