Promote the editor to a first-class v3 sidebar item (#546)

The Arrangement Editor plugin was only reachable via the generic Plugins
gallery. Give it a dedicated sidebar entry (Library group, below Songs)
through the existing PROMOTED_PLUGINS mechanism in shell.js — a NAV entry,
a promoted slot anchored after "songs", and an edit icon.

renderPromotedNav already gates each promoted slot on the plugin being
present in /api/plugins, so the entry shows only when the editor is
installed. The displayed label comes from the plugin manifest's nav.label.

Signed-off-by: topkoa <topkoa@gmail.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

static/v3/shell.js

@@ -42,6 +42,7 @@
         // plugin- guard applies.
         { key: 'slopscale',   screen: 'plugin-slopscale',   label: 'SlopScale - 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' },
         // Not in the sidebar groups, but routable (profile badge → here).
         { key: 'profile',   screen: 'v3-profile',   label: 'Profile',         group: null,      icon: 'user' },
     ];
@@ -52,6 +53,7 @@
     const PROMOTED_PLUGINS = [
         { navKey: 'slopscale',   pluginId: 'slopscale',   slotId: 'v3-nav-slopscale',    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' },
     ];
     const TOPBAR_KEYS = ['home', 'songs', 'plugins', 'settings'];
     const SIDEBAR_GROUPS = ['HOME', 'LIBRARY'];
@@ -72,6 +74,7 @@
         tag: 'M20.6 13.4l-7.2 7.2a2 2 0 01-2.8 0l-7-7V4h9.6l7.4 7.4a2 2 0 010 2zM7.5 7.5h.01',
         amp: 'M4 5h16a1 1 0 011 1v12a1 1 0 01-1 1H4a1 1 0 01-1-1V6a1 1 0 011-1zm11 4a3 3 0 100 6 3 3 0 000-6zM6.5 8.5h.01M9 8.5h.01',
         target: 'M12 3a9 9 0 100 18 9 9 0 000-18zm0 4a5 5 0 100 10 5 5 0 000-10zm0 4a1 1 0 100 2 1 1 0 000-2z',
+        edit: 'M11 4H4a2 2 0 00-2 2v14a2 2 0 002 2h14a2 2 0 002-2v-7M18.5 2.5a2.1 2.1 0 013 3L12 15l-4 1 1-4 9.5-9.5z',
     };
     function iconSvg(name) {
         const d = ICONS[name] || ICONS.disc;