feat(qt): Excel-style multi-select — Shift ranges across every surface

Selection existed but had no keyboard: every click was a single toggle,
so picking 40 tracks meant 40 clicks. Slint and Tauri have had ranges
since 2026-06; this is that port.

NINE surfaces, not the five the track views suggested: Album, Artist,
Playlist, Label, Local Album, plus Library's tracks and albums and Local
Library's tracks and albums. Each carried its own hand-copied `selected`
map and its own `toggleSelected`, so the rule went into
controls/SelectionModel.qml and the maps stayed where they were — the
smallest change that stops it being copied a tenth time.

The rules are Tauri's `applyShiftRange`, unchanged: a plain click
toggles one row without clearing the rest, Shift fills the range from
the anchor ADDITIVELY (so re-dragging a range cannot eat the set) and
leaves the anchor where it is, and Ctrl/Cmd is the same as a plain click
— not a shortcut, but the behaviour both references already have,
because an accumulating click is what "add without disturbing" means.

Qt makes this CHEAPER than Slint, and the difference is worth recording:
Slint had no modifiers at click time and had to read a thread-local set
from a ModifiersChanged handler, with a documented stale-read edge case
when the window lost focus mid-chord. A QML MouseArea delivers
`mouse.modifiers` on the event itself, so the modifier and the click can
never disagree and no such seam exists here.

Three judgement calls, stated because they are not forced:

  - Library gets TWO models, one per kind. Its rows are mixed, and one
    anchor would let a Shift-click on a track sweep up the albums drawn
    between it and the anchor.
  - Ranges run over the VISIBLE rows. Local Library's search box is a
    view filter, and a range over rows the user cannot see is not the
    range they asked for — the same call `select-all` already made.
  - The Local Library tree rail keeps plain toggles. Its selection lives
    in Rust (QbzLocal.treeToggle*Select), so there is no QML map to hang
    an anchor off; teaching the Rust side about an anchor is its own
    change, and the file says so where a reader would expect otherwise.

The anchor is an ID, not an index, because every one of these views can
re-sort or filter under a live selection — an index anchor silently
points at a different track the moment the order changes. Slint's
selection.rs made the same call for the same reason.

Verified by driving the app: in Abbey Road, click row 1 then Shift-click
row 5 selects exactly rows 1-5 and leaves 6-8 untouched, with the bar
reading "5 selected"; the same in a playlist, additively over a row that
was already selected.

cc

crates/qbz-qt/qml/rows/TrackRow.qml

@@ -136,7 +136,10 @@ Rectangle {
     /// existing call site is untouched.
     property bool selectMode: false
     property bool checked: false
-    signal toggleSelect()
+    /// `modifiers` is the mouse event's, straight through — Shift is what
+    /// turns a click into a range (controls/SelectionModel.qml). Hosts that
+    /// do not care can keep a no-argument handler; QML allows it.
+    signal toggleSelect(int modifiers)
     // Per-row artwork placeholder (see the 36px cell below). The host view
     // owns the phase clock so one timer drives every row.
     property bool artPending: false
@@ -677,7 +680,7 @@ Rectangle {
                     anchors.fill: parent
                     hoverEnabled: true
                     cursorShape: Qt.PointingHandCursor
-                    onClicked: root.toggleSelect()
+                    onClicked: root.toggleSelect(Qt.NoModifier)
                 }
             }
             Text {
@@ -1473,7 +1476,7 @@ Rectangle {
             // In select mode the WHOLE row body is a selection target
             // (TrackRow.slint:174) — never a play.
             if (root.selectMode) {
-                root.toggleSelect()
+                root.toggleSelect(mouse.modifiers)
                 mouse.accepted = true
                 return
             }