Fix cover mode, add model selection (turbo/sft/base), add Lego mode

[Copilot]

Cover mode was broken due to a non-existent --cover-strength CLI flag. Additionally, wires in model selection and Lego mode support.

Bug fix: cover mode

• Remove --cover-strength CLI arg (unsupported by ace-synth)
• Rename JSON field cover_strength → audio_cover_strength
• --src-audio is retained as the correct CLI mechanism; strength is JSON-only

{
  "caption": "Jazz piano cover with brushed drums and walking bass",
  "lyrics": "[Instrumental]",
  "audio_cover_strength": 0.5
}

Model selection (turbo / sft / base)

• DitModel { Turbo, Sft, Base } enum on the processor; persisted in config.xml and DAW project state
• Model combo in Generate tab with onChange persistence; maps to acestep-v15-{turbo,sft,base}-Q8_0.gguf

Lego mode

• Third generation mode alongside Text-to-Music and Cover — adds "lego": "<track>" to the JSON request and passes the backing track via --src-audio
• Instrument dropdown: vocals, backing_vocals, drums, bass, guitar, keyboard, percussion, strings, synth, fx, brass, woodwinds
• Base model is required for lego; combo auto-locks to Base when lego mode is selected
• Refactored setCoverMode(bool) → setGenerationMode(GenerationMode) enum (TextToMusic, Cover, Lego)

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