feat(qbz-qt): Local Library — folder tree, windowed album grid, detail

The sidebar has had a "Local Library" row since the shell was ported and
nothing behind it: the nav entry did not even navigate. This builds the view.

QbzLocal is its own domain bridge (the phase-23 pattern, one #[cxx_qt::bridge]
per domain) over local_library_qt, which consumes qbz-library for the scanning,
the folder-vs-tag album identity and the queries rather than reimplementing any
of it. The view carries the folder tree, the album grid, album detail, the
track tabs, sorting and search, plus the scanning and empty states.

Windowed from the start, not as a later pass: real local libraries reach 16k
tracks and that is exactly the surface where the Slint port freezes. Artwork
is id-keyed and evicted with the window.

Playback is source-aware — a local track plays from disk through the player's
play_data seam, because the Qobuz tier-walk needs a client and fails on every
local advance. TrackRow gains an opt-out for its drag source: a local row's id
is a local database row, and the sidebar's drop handler forwards whatever it
receives to `playlist add-tracks` as a Qobuz catalog id, so dragging one onto a
playlist would have silently added an unrelated track.

Not included, and tracked as follow-ups: Plex union and thumbnails, multi-select
and the bulk bars, ephemeral folders, the tag editor, A-Z jump strips, the
group-by dropdown, the tree's resize handle, per-tab scroll restore, and local
favourites on rows. Album detail renders inside the view rather than as its own
routed page.

cc

crates/qbz-qt/qml/shell/Sidebar.qml

@@ -180,7 +180,10 @@ Rectangle {
                 navId: "local"
                 name: "hard-drive"
                 label: QbzSession.tr("Local Library", QbzSession.trRev)
-                onClicked: root.activeNav = "local"
+                onClicked: {
+                    root.activeNav = "local"
+                    QbzShell.navigateTo("local")
+                }
             }
             NavRow {
                 navId: "myqbz"