feat(slint): playlist detail polish, sort (incl. custom) + WIP track drag

Polish + consistency:
  - Compact playlist header (cover 150px; nav->header spacing
    homologated with AlbumView); description truncated to ~2 lines +
    "Read more" modal; custom-artwork buttons only on cover hover.
  - New Typography.section (18px) token for hero/section titles,
    applied across Album / Artist / Mix / Playlist + Home carousels.
  - Shorter track rows app-wide (50px row, 36px artwork, -1 fonts);
    +3px column-header/divider gap; wider track-number column.
  - Reusable ExpandableSearch (the JumpNavBar page-search pattern:
    magnifier -> opens left -> X to close) in the playlist action row.
  - Custom ListScrollbar (thicker, min thumb height, own gutter) since
    Slint's ListView scrollbar can't be styled.

Sort:
  - Default / Title / Artist / Album / Duration with asc/desc per
    field, plus Custom (manual order persisted in library.db with
    up/down reorder). Client-side search + sort combined.

WIP: track drag-and-drop onto sidebar playlists
  - DragState/DragActions globals, drag source on the shared TrackRow,
    follow-cursor DragGhost, sidebar playlist rows self-detect the drop
    via absolute-position, multi-select group drag, add_tracks_to_playlist
    on drop, Flickables freeze scroll during a drag.
  - KNOWN ISSUE: the ghost is offset from the cursor (the row
    absolute-position + pointer mapping is off, esp. inside Flickables).
    Diagnostic logging is left in (TrackRow debug() + a start log) to
    pin down the wrong term before the generic fix.

cc

crates/qbz-slint/ui/artist/ArtistPageView.slint

@@ -17,6 +17,7 @@ import {
     ContentView,
     NetworkSidebarState,
     NetworkSidebarActions,
+    DragState,
 } from "../state.slint";
 import { QbzIcon } from "../primitives/QbzIcon.slint";
 import { TrackRow } from "../primitives/TrackRow.slint";
@@ -159,6 +160,9 @@ export component ArtistPageView inherits Rectangle {
     // it can stay anchored on the right and fill the available height
     // independently of the page scroll.
     page-flickable := Flickable {
+        // Freeze scrolling while a track drag is in flight, so the
+        // dragged row (and thus the ghost position) doesn't drift.
+        interactive: !DragState.active;
         viewport-width: self.width;
         viewport-height: page.preferred-height;
 
@@ -290,7 +294,7 @@ export component ArtistPageView inherits Rectangle {
                     Text {
                         text: ArtistState.name;
                         color: Theme.text-primary;
-                        font-size: Typography.title;
+                        font-size: Typography.section;
                         font-weight: Typography.bold;
                         overflow: elide;
                     }
@@ -534,7 +538,7 @@ export component ArtistPageView inherits Rectangle {
 
             for track[index] in ArtistState.top-tracks: TrackRow {
                 visible: root.top-tracks-expanded || index < root.top-tracks-preview;
-                height: self.visible ? 56px : 0px;
+                height: self.visible ? 50px : 0px;
                 track: track;
                 index: index;
                 // Artist Popular Tracks: number cell (no cover), with