feat(myqbz): collapsible My QBZ sidebar with per-element hide and toolbar

Opt-in collapsible My QBZ tree in the sidebar (Settings > Navigation): a
branding-titled parent over Mixtapes and Collections sections, each header a
link to its grid with a chevron that only collapses. Header text and icon on
the left, a Playlists-parity toolbar floating right (search, new, sort A-Z/Z-A,
refresh, collapse) with the chevron aligned to the playlist carets. The tree
feed rides QbzShell (QbzMyQbz is not reliably registered in the shell context)
and is published through publish_sidebar, so it lands reliably at startup.

Per-element visibility reuses the mixtape hidden flag (repo::set_hidden): a
row's right-click hides it from the sidebar, and the detail hero menu toggles
"Hide from sidebar" / "Show in sidebar". Collapse state for the playlist tree
and all three My QBZ sections is persisted. The collapsible tree and the flat
My QBZ entry are mutually exclusive — the classic row is suppressed whenever the
tree is on. Settings gains a Navigation section (Navigation + Search moved out
of Appearance). Detail view also gains a discreet, column-aligned sub-header
over the inline track list. Eight locales updated.

cc

crates/qbz-qt/build.rs

@@ -637,6 +637,7 @@ fn main() {
                 "qml/rows/TrackListHeader.qml",
                 "qml/rows/TrackRow.qml",
                 "qml/settings/AppearanceSettings.qml",
+                "qml/settings/NavigationSettings.qml",
                 "qml/settings/CustomThemeEditor.qml",
                 "qml/settings/IntegrationsSettings.qml",
                 "qml/settings/SettingsView.qml",
@@ -689,6 +690,7 @@ fn main() {
                 "qml/shell/Sidebar.qml",
                 "qml/shell/SidebarFolderFlyout.qml",
                 "qml/shell/SidebarRowMenu.qml",
+                "qml/shell/MyQbzSidebarMenu.qml",
                 "qml/shell/SidebarNowPlayingDock.qml",
                 "qml/shell/AudioSettingsMenu.qml",
                 "qml/shell/AlbumQuickView.qml",