feat(qt): playlist manager, folders, importer and the Library parity round

Ports the whole playlist-organisation domain, which had no counterpart in
this frontend, plus the Library work the owner asked not to defer.

Playlist Manager. New controller (playlist_manager_qt, _rows, _ops) over the
already-ported folders_qt repository, which until now had zero consumers.
Grid, list and folder-tree modes; sort, filter and search; folder cards and
chips; drag reorder and up/down arrows. Reached from the sidebar's "..."
menu, whose New folder / Manage playlists / Import rows had been rendering
dimmed and inert behind a comment saying the seam did not exist.

Folders. QbzFolderEdit plus the create/edit modals and PmFolderIcon: preset
or custom-image icons, colour, hidden flag, delete. Deleting a folder nulls
its LOCAL members' folder_id explicitly, because the app keeps the
foreign_keys pragma off, so the schema's ON DELETE SET NULL never fires for
them.

QbzPlaylistEdit. One shared rename/description/delete editor replacing the
inline popup in PlaylistView. Three corrections the old path had wrong: the
description is carried through a rename instead of being written as NULL,
a FOLLOWED playlist routes to unsubscribe (playlist/delete returns 200 and
silently no-ops on a playlist you do not own), and the post-delete back
navigation is conditional on actually being on that playlist's page, so
deleting from the manager no longer navigates you off it.

Importer. Spotify / Apple Music / Tidal / Deezer, over the shared
qbz-playlist-import crate.

Picker local mode. The reference's four-case matrix — target local or Qobuz,
payload local refs or Qobuz ids. Local playlists list unconditionally and
Qobuz ones only when online. Local refs onto a Qobuz playlist write the
library.db sidecar at next_playlist_sidecar_position(pid, qobuz_count); the
0-based enumerate the reference used before collided slots and lost rows.
The carried payload can no longer be parsed as a catalog id when it is a
library row id. Unblocks the local bulk add-to-playlist.

Library. A-Z jump strip, Albums list mode, Artists side panel, per-tab
persistence and multi-select. Preferences land in the same
<data-dir>/qbz/favorites_ui.json the shipping build writes, so switching
between the two frontends keeps them. LibraryView is split into
views/library/ on the way past 1800 lines.

Local library. Group-by now persists; playback queues the order on screen
rather than the SQL page order; switching album-identity mode invalidates
the Artists tab; artist matching folds diacritics and compares normalized
credit parts, so "Air" stops listing "Airbourne" and "Beyonce" stops being
two artists.

Offline. activate_offline never persisted a last-user marker, so every
per-user path resolved to None and all local reads came back empty; a
session that started offline saw no local playlists at all. The sidebar's
load-once latch was function-local and therefore unreachable from the logout
reset, so a second login never rebuilt the tree. Both are session-scoped
now, and the sidebar Refresh row runs the local-safe verb instead of being
dimmed with no way to recover.

Also fixes, found while verifying rather than reported: artwork derivatives
were written straight to their final path, so a QML Image resolving one
mid-encode opened a truncated png and Qt cached that failure against the URL
for the rest of the session; Library's track group separators referenced a
Component declared in another delegate's scope and could never render; the
genre filter was drawn on Tracks and Albums but applied only on All; an
All-feed row called a player method that does not exist; and the sidebar's
drop-hot highlight used Slint's #RRGGBBAA literals in Qt's #AARRGGBB slot.

17 new msgids across all 8 locales.

cc

crates/qbz-qt/qml/controls/QbzAlphaStrip.qml

@@ -0,0 +1,61 @@
+// Vertical A-Z jump strip (favorites/FavoritesView.slint:398 `AlphaStrip`;
+// LocalLibraryView.slint:577 is a 1:1 lift of the same component). 18px
+// gutter, 15px rows, 9px semibold muted letters that go accent on hover.
+// Emits jump(ordinal, index); the host scrolls its own view — exactly like
+// the Slint, which scrolls proportionally on the grouped grids (:1996) and by
+// row index on the uniform track list (:1687).
+//
+// PROMOTED from views/local/AlphaStrip.qml, unchanged except for the import
+// depth — same move QbzMultiSelectBar made and for the same reason: the Slint
+// component was always shared (Local Library albums/artists/tracks AND the
+// Library favorites tabs), so the "local" folder was a naming accident. The
+// old views/local/AlphaStrip.qml is GONE; this file is the only copy.
+// Call sites: views/local/{LocalAlbumsTab,LocalArtistsTab,LocalTracksTab}.qml,
+// views/library/{LibraryAlbumsList}.qml and views/LibraryView.qml.
+
+import QtQuick
+import com.blitzfc.qbz
+import "../theme"
+
+Item {
+    id: root
+
+    /// [{ letter, index }] — index = the row ordinal in the flat model.
+    property var jumps: []
+    signal jump(int ordinal, int index)
+
+    QbzTheme { id: theme }
+
+    width: 18
+
+    Column {
+        anchors.centerIn: parent
+        spacing: 0
+        Repeater {
+            model: root.jumps
+            delegate: Item {
+                id: cell
+                required property var modelData
+                required property int index
+                width: 18
+                height: 15
+                Text {
+                    anchors.fill: parent
+                    text: cell.modelData.letter
+                    color: cellArea.containsMouse ? theme.accent : theme.textMuted
+                    font.pixelSize: 9
+                    font.weight: theme.weightSemibold
+                    horizontalAlignment: Text.AlignHCenter
+                    verticalAlignment: Text.AlignVCenter
+                }
+                MouseArea {
+                    id: cellArea
+                    anchors.fill: parent
+                    hoverEnabled: true
+                    cursorShape: Qt.PointingHandCursor
+                    onClicked: root.jump(cell.index, cell.modelData.index)
+                }
+            }
+        }
+    }
+}