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/foundation/typography.slint

@@ -6,6 +6,10 @@ export global Typography {
     out property <length> body: 15px;
     out property <length> subtitle: 15px;
     out property <length> button: 17px;
+    // Canonical page / section title — Home section headings ("New
+    // Releases", ...) and the hero titles on Album / Artist / Playlist
+    // / Mixtape detail pages. One size for all, for consistency.
+    out property <length> section: 18px;
     out property <length> heading: 19px;
     out property <length> title: 25px;
     out property <length> wordmark: 29px;