- RNNoise (AI Noise Suppression) now enabled by default for all users
- Remove redundant browser Noise Suppression toggle from VoiceControls
and UserSettings — AudioManager handles it automatically as fallback
- Add AI Noise Suppression toggle to UserSettings panel
- Rename toggleRnnoise → setRnnoiseEnabled for clearer API
- Store migration v3→v4: enable RNNoise for existing users
- Keep Echo Cancellation and Auto Gain Control (orthogonal features)