`set wifi.powersave min` stored 0 and applied WIFI_PS_MIN_MODEM, but the
bridge's post-reconnect mapping read the same 0 as WIFI_PS_NONE. A node
explicitly configured for minimum modem sleep therefore ran with power save off
after its first reconnect, while `get wifi.powersave` still reported min. The
stored default is already 1 (`none`), so nothing here changes the product
default — it stops an operator's explicit choice from being reinterpreted.
Move the value/name/mode table into WifiPowerSavePolicy (pure, host-tested) and
use it from the CLI setter and getter, the web config snapshot and the bridge.
The bridge now also applies the mode when it finds the STA already associated at
start — that path has no connect transition to carry the setting, so a bridge
restart used to leave whatever mode was set before.
