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/MixView.qml

@@ -28,13 +28,16 @@
 // rows/TrackCols.qml for both.
 //
 // POC-NOTEs, each deliberate and each with a precedent in this port:
-// - NO multi-select bar. qml/rows/TrackRow.qml has no multi-select arm, and
-//   one of the .slint bar's seven actions (add-to-playlist) still has no
-//   bridge seam at all — LabelView.qml made the same call. (add-to-mixtape
-//   IS live now: the per-row menu reaches QbzMyQbzAdd.open; it is only the
-//   BULK form that is missing, for want of a selection.) The .slint header's
-//   select disc renders DIMMED and inert here (the ArtistView / LabelView
-//   precedent).
+// - NO multi-select bar — and the ORIGINAL reason is now STALE on both
+//   halves: rows/TrackRow.qml:116 DOES have the `selectMode` arm, and
+//   add-to-playlist DOES have a bridge seam (QbzPlaylistPicker.openForTracks,
+//   src/playlist_picker_bridge.rs). What is still missing here is only the
+//   selection STATE and the bar itself — LibraryView.qml's `tracksSelected`
+//   map + qml/controls/MultiSelectBar-style host is the pattern to copy.
+//   Until then the .slint header's select disc renders DIMMED and inert here
+//   (the ArtistView / LabelView precedent). Per-ROW add-to-playlist and
+//   add-to-mixtape are both LIVE on this page already (the shared TrackRow
+//   menu).
 // - The "Add to playlist" disc (list-plus) is inert in the .slint too — it
 //   carries no `clicked` there (MixView.slint:182-188) — so it is inert
 //   here, dimmed the same way, rather than silently absent.