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/search/SearchResultsView.slint

@@ -8,7 +8,7 @@
 // per-type tabs show the full loaded set as a card grid or row list with
 // a load-more button.
 
-import { SearchState, SearchActions, TrackItem, SearchPlaylistItem, AlbumCardItem, SlimItem } from "../state.slint";
+import { SearchState, SearchActions, TrackItem, SearchPlaylistItem, AlbumCardItem, SlimItem, DragState } from "../state.slint";
 import { Theme } from "../foundation/semantic-colors.slint";
 import { Typography } from "../foundation/typography.slint";
 import { Radius } from "../foundation/radius.slint";
@@ -420,6 +420,7 @@ export component SearchResultsView inherits Rectangle {
 
         // --- Scrollable results body ------------------------------------
         Flickable {
+            interactive: !DragState.active;
             vertical-stretch: 1;
             viewport-width: self.width;
             viewport-height: body.preferred-height;