docs: update README and website for JUCE migration

Rewrite all documentation to reflect the v1.0 JUCE-based architecture:
- JUCE framework replaces Dear ImGui and CLAP-wrapper
- FLAC lossless codec alongside Vorbis
- Multichannel output routing (17 stereo buses)
- DAW transport sync with live BPM/BPI changes
- Standalone application mode
- Linux support (new platform)
- Updated build instructions, architecture, and threading model
- Updated all 5 website pages (index, features, download, docs, FAQ)

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

Author: mkschulze