fix(init): apply UseInterfaces from global defaults correctly

The UseInterfaces flag had default:"true", so the condition
`!options.UseInterfaces` was never true - meaning global defaults
could never set UseInterfaces=false.

Fix by always applying the global defaults value for UseInterfaces,
making `wails3 setup` the authoritative source for this preference.

Addresses: https://github.com/wailsapp/wails/pull/4906#discussion_r2980562751

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>