feat: Move audio driver setting from config.h to keyboard.json Form Editor

[yoichiro]

Summary

• Remove AUDIO_DRIVER from config.h Form Editor (QMK ignores it in config.h)
• Add Audio tab to keyboard.json Form Editor with audio.driver selection
  • FreeSolo autocomplete: select from pwm_hardware, pwm_software, dac_basic, dac_additive or type a custom value
• Tab order: General → USB/MCU → Hardware → Features → Lighting → Audio → Encoder

Test plan

• Open config.h → Form Editor → Audio tab → verify AUDIO_DRIVER field is gone
• Open keyboard.json → Form Editor → verify Audio tab exists between Lighting and Encoder
• Select "pwm_hardware" from dropdown → switch to Code Editor → verify "audio": { "driver": "pwm_hardware" } in JSON
• Type custom driver value → verify it appears in JSON
• Clear driver value → verify audio object is removed from JSON
• Verify Encoder tab still works at index 6

🤖 Generated with Claude Code