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/views/playlistmanager/PmFolderCard.qml

@@ -0,0 +1,120 @@
+// PmFolderCard — the GRID-mode folder cell (PlaylistManagerView.slint:291-373).
+// 160 × 150, r10, elevated → surface-hover on body hover, 0.6 opacity while the
+// folder is hidden (the ONLY hidden affordance on a folder — no eye glyph, no
+// badge).
+//
+// BOTH click targets open the folder EDITOR (contract D7): the reference wires
+// `open =>` and `edit =>` to the SAME callback, `edit-folder(folder.id)`
+// (`:1364-1365`). You cannot enter a folder from the grid — folder navigation
+// is the tree. Do not "fix" this into a drill-down.
+//
+// Hover semantics are the reference's OR (`ta.has-hover || edit-ta.has-hover`,
+// `:349`): the pencil's own MouseArea sits above the card's and would otherwise
+// steal `containsMouse`, dropping the pencil to opacity 0 under the pointer
+// while it still accepts the click. Declaration order is z-order in QML and
+// `propagateComposedEvents` does not rescue `pressed`, so the pencil is
+// declared AFTER the card's MouseArea, deliberately.
+
+import QtQuick
+import com.blitzfc.qbz
+import "../../controls"
+import "../../theme"
+
+Rectangle {
+    id: root
+
+    /// One `QbzPlaylistManager.foldersJson` entry (contract §4.1).
+    property var folder: ({})
+    /// Emitted by the body AND by the pencil — one action, two targets (D7).
+    signal openRequested()
+
+    QbzTheme { id: theme }
+
+    readonly property bool hot: cardArea.containsMouse || pencilArea.containsMouse
+
+    width: 160
+    height: 150
+    radius: 10
+    color: cardArea.containsMouse ? theme.surfaceHover : theme.surfaceElevated
+    opacity: (root.folder.isHidden === true) ? 0.6 : 1.0
+
+    // padding 16 / spacing 8 / centred column.
+    Column {
+        anchors.centerIn: parent
+        width: parent.width - 32
+        spacing: 8
+
+        PmFolderIcon {
+            anchors.horizontalCenter: parent.horizontalCenter
+            size: 64
+            iconPreset: root.folder.iconPreset || "folder"
+            iconColor: root.folder.iconColor || ""
+            hasColor: root.folder.hasColor === true
+            hasCustomImage: root.folder.hasCustomImage === true
+            customImagePath: root.folder.customImagePath || ""
+        }
+        Text {
+            width: parent.width
+            height: 18
+            text: root.folder.name || ""
+            color: theme.textPrimary
+            font.pixelSize: 13
+            font.weight: theme.weightMedium
+            horizontalAlignment: Text.AlignHCenter
+            verticalAlignment: Text.AlignVCenter
+            elide: Text.ElideRight
+        }
+        Text {
+            width: parent.width
+            // Rust never pluralises this one — the reference picks the string in
+            // the markup (`:332-334`), so the two msgids are resolved here. An
+            // empty folder reads "0 playlists"; there is no empty state.
+            text: (root.folder.count === 1)
+                ? QbzSession.tr("1 playlist", QbzSession.trRev)
+                : QbzSession.tr("{} playlists", QbzSession.trRev)
+                    .replace("{}", root.folder.count || 0)
+            color: theme.textMuted
+            font.pixelSize: 12
+            horizontalAlignment: Text.AlignHCenter
+            elide: Text.ElideRight
+        }
+    }
+
+    MouseArea {
+        id: cardArea
+        anchors.fill: parent
+        hoverEnabled: true
+        cursorShape: Qt.PointingHandCursor
+        onClicked: root.openRequested()
+    }
+
+    // Hover pencil, top-right. Declared after the card area so it wins the
+    // press (see the header).
+    Rectangle {
+        x: parent.width - width - 8
+        y: 8
+        width: 24
+        height: 24
+        radius: 4
+        opacity: root.hot ? 1.0 : 0.0
+        Behavior on opacity { NumberAnimation { duration: 150 } }
+        // Goes DARKER than the card on its own hover — deliberate upstream
+        // (`:351`).
+        color: pencilArea.containsMouse ? theme.surfaceCard : theme.surfaceElevated
+
+        QbzIcon {
+            anchors.centerIn: parent
+            name: "pen-line"
+            width: 12
+            height: 12
+            tintName: pencilArea.containsMouse ? "textPrimary" : "muted"
+        }
+        MouseArea {
+            id: pencilArea
+            anchors.fill: parent
+            hoverEnabled: true
+            cursorShape: Qt.PointingHandCursor
+            onClicked: root.openRequested()
+        }
+    }
+}