luci-app-tailscale-community: fix default state file path

luci-app-tailscale-community: fix default state file path

Align default state file path with `tailscale` package.

When `tailscale.settings` doesn't exist in UCI, this LuCI App tries to
create one with settings parsed from the state file. The parser first
looks for `tailscale.settings.state_file` but this value sure wouldn't
exist under the circumstances and will fall back to default path.

Fix the default path for the parser to locate the state file properly.

Signed-off-by: Kuan-Yi Li <redacted>