Serve the Pi's web UI on port 80 instead of 8080

Just http://<pi-ip> now instead of http://<pi-ip>:8080. Safe since
the systemd service already runs as root (needed for GPIO/SPI access
anyway), so binding the privileged port isn't a new requirement.

Local --mock dev stays on 8080 (README/main.py default unchanged) --
that's normally run without root, and port 80 needs it.

install.sh now does enable + restart instead of enable --now, so
re-running it after a unit-file change (like this one) actually
applies on an already-installed instance, not just a fresh install.

README.md

@@ -45,7 +45,7 @@ InkyPi (or anything else) already running, it stops it first -- two things
 can't drive the same panel at once -- but nothing is copied from or reused
 out of it. Safe to re-run.
 
-Then open `http://<pi-ip>:8080`, fill in Navidrome credentials under
+Then open `http://<pi-ip>`, fill in Navidrome credentials under
 **Navidrome** and a city under **Weather**. Everything saves immediately and
 takes effect within a few seconds -- no restart needed.