Add the bundled desktop `audio_engine` plugin (native audio I/O, VST
hosting, amp modeling, pitch detection) as a dedicated v3 sidebar entry
in the HOME group, immediately below Settings, via the existing
PROMOTED_PLUGINS mechanism in static/v3/shell.js. Gated on the plugin
being installed (renderPromotedNav checks /api/plugins), so it shows
only in desktop builds. Adds a new `audio` waveform icon; the displayed
label comes from the plugin manifest nav.label ("Audio").
Test: tests/browser/audio-engine-nav.spec.ts (placement after Settings,
inside the HOME group, links to the plugin screen).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
