sni: make gRPC address configurable via O2_SNI_GRPC_ADDR env var

[Algo-17]

This is a simple fix to allow connecting to an SNI service on another machine, tested with the SNI-enabled SNES core for the Mister FPGA. Default behavior should be unaffected.

Here is the beginning of the log showing it connecting to the Mister.

$ go build -o o2 ./webui/o2 && O2_SNI_GRPC_ADDR=192.168.17.50:8191 O2_RETROARCH_DISABLE=1 O2_FXPAKPRO_DISABLE=1 ./o2
2026/04/22 13:24:19 Read env var O2_FXPAKPRO_DISABLE: using '1'
2026/04/22 13:24:19 disabling fxpakpro snes driver
2026/04/22 13:24:19 Read env var O2_MOCK_ENABLE: not found; defaulting to '0'
2026/04/22 13:24:19 Read env var O2_DEFAULT_SERVER_PORT: not found; defaulting to '4590'
2026/04/22 13:24:19 Read env var O2_RETROARCH_DISABLE: using '1'
2026/04/22 13:24:19 disabling retroarch snes driver
2026/04/22 13:24:19 Read env var O2_QUSB2SNES_DISABLE: not found; defaulting to '1'
2026/04/22 13:24:19 disabling qusb2snes snes driver
2026/04/22 13:24:19 Read env var O2_SNI_DISABLE: not found; defaulting to '0'
2026/04/22 20:24:19.537897 logging to '/var/folders/nv/40vlvmjd4dg3qytsyc2z8x9m0000gn/T/o2-2026-04-22T20-24-19-537Z.log'
2026/04/22 20:24:19.537900 o2 v0.0.0 dirty built on 2021-05-03T00:17:00Z by go
2026/04/22 20:24:19.537940 Read env var O2_WEB_LISTEN_HOST: not found; defaulting to '0.0.0.0'
2026/04/22 20:24:19.537945 Read env var O2_WEB_LISTEN_PORT: not found; defaulting to '27637'
2026/04/22 20:24:19.537948 Read env var O2_WEB_BROWSER_HOST: not found; defaulting to '127.0.0.1'
2026/04/22 20:24:19.538193 Read env var O2_SNI_GRPC_ADDR: using '192.168.17.50:8191'
2026/04/22 20:24:19.544677 viewmodel: loadConfiguration: loading...
2026/04/22 20:24:19.544930 viewmodel: snesconnected: open: driver='sni', device='mister:://mister'
2026/04/22 20:24:19.544987 notify: Connected to SNES

Here is a screenshot of the webpage showing the connection and the initial few updates of a session running on the Mister.