feat(v3): promote Audio Engine to a first-class sidebar entry (after Settings) (#613)

The desktop Audio Engine plugin (input device selection, VST hosting, pitch
detection, and the new config Reset/repair UI) was reachable only via the
generic Plugins gallery — per-plugin manifest nav entries aren't surfaced in
the v3 sidebar unless the plugin is promoted. Add it to PROMOTED_PLUGINS
anchored after Settings, plus the matching NAV registry entry so the slot
resolves its label/screen. Desktop-only by construction: the slot is filled
only when /api/plugins reports audio_engine installed, so the web app shows
no dead entry.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

static/v3/shell.js

@@ -50,6 +50,7 @@
         { key: 'virtuoso',    screen: 'plugin-virtuoso',    label: 'Virtuoso - Practice', group: null, icon: 'target' },
         { key: 'rig_builder', screen: 'plugin-rig_builder', label: 'Rig Builder', group: null,      icon: 'amp' },
         { key: 'editor',      screen: 'plugin-editor',      label: 'Song Editor', group: null,      icon: 'edit' },
+        { key: 'audio_engine', screen: 'plugin-audio_engine', label: 'Audio', group: null,        icon: 'amp' },
         // Not in the sidebar groups, but routable (profile badge → here).
         { key: 'profile',   screen: 'v3-profile',   label: 'Profile',         group: null,      icon: 'user' },
     ];
@@ -61,6 +62,7 @@
         { navKey: 'virtuoso',    pluginId: 'virtuoso',    slotId: 'v3-nav-virtuoso',    anchorAfter: 'feedbarcade' },
         { navKey: 'rig_builder', pluginId: 'rig_builder', slotId: 'v3-nav-rig-builder', anchorAfter: 'saved' },
         { navKey: 'editor',      pluginId: 'editor',      slotId: 'v3-nav-editor',     anchorAfter: 'songs' },
+        { navKey: 'audio_engine', pluginId: 'audio_engine', slotId: 'v3-nav-audio-engine', anchorAfter: 'settings' },
     ];
     const TOPBAR_KEYS = ['home', 'songs', 'plugins', 'settings'];
     const SIDEBAR_GROUPS = ['HOME', 'LIBRARY'];