feat(slint): LocalLibrary ephemeral folders + Folders tree 1:1 port

Ephemeral folders: open / browse / play a folder without indexing it —
album-grouped pane, CUE seek fast-path (no full re-decode per virtual track),
folder-path persistence + rehydrate, "already playing" add-vs-replace choice
dialog (ephemeral-only), and now-playing / queue / menu gating so an ephemeral
track exposes no persistence actions.

Folders tree brought 1:1 with Tauri:
- Rail header: select-mode, collapse-all, open-ephemeral, tree search.
- Resizable rail (default ~20% narrower; drag handle, min 50% default /
  max 50% of the content area).
- Subfolder cover grid + grid/list toggle, recursive track count, name search;
  compact icon bulk bar (folder-check / play-next / queue / playlist / clear).
- Tree is the default mode; circular checkboxes homologated with TrackRow;
  compact rows (smaller type + icon, no ambiguous trailing count).
- Flat/Tree icon toggle (disc-album / folder-tree), floated right.

Per-source TrackRow context menu (logic, not a blanket trim): Qobuz/offline =
full; local = play/next/queue + add-to-playlist; plex/ephemeral = play/next/
queue only.

App-wide standard: Math.round pixel-snap on icon centering across the UI to
kill sub-pixel blur (CircleAction, header back/forward/collapse, and the rest).
NowPlayingBar context icon -> turntable (frees disc-album for the Folders
toggle). New assets: folder-check, folder-tree, turntable.

cc

crates/qbz-slint/ui/primitives/TrackPlayCell.slint

@@ -97,8 +97,8 @@ export component TrackPlayCell inherits Rectangle {
             source: @image-url("../assets/icons/music.svg");
             width: root.art-size * 0.45;
             height: root.art-size * 0.45;
-            x: (parent.width - self.width) / 2;
-            y: (parent.height - self.height) / 2;
+            x: Math.round((parent.width - self.width) / 2 / 1px) * 1px;
+            y: Math.round((parent.height - self.height) / 2 / 1px) * 1px;
             tint: Theme.text-muted;
         }
         Image {
@@ -115,8 +115,8 @@ export component TrackPlayCell inherits Rectangle {
             animate opacity { duration: 150ms; }
         }
         if root.show-bars: EqualizerBars {
-            x: (parent.width - self.width) / 2;
-            y: (parent.height - self.height) / 2;
+            x: Math.round((parent.width - self.width) / 2 / 1px) * 1px;
+            y: Math.round((parent.height - self.height) / 2 / 1px) * 1px;
         }
         if root.show-overlay && !root.show-bars: QbzIcon {
             source: (root.is-playing && root.row-hovered)
@@ -124,8 +124,8 @@ export component TrackPlayCell inherits Rectangle {
                 : @image-url("../assets/icons/play-fill.svg");
             width: 20px;
             height: 20px;
-            x: (parent.width - self.width) / 2;
-            y: (parent.height - self.height) / 2;
+            x: Math.round((parent.width - self.width) / 2 / 1px) * 1px;
+            y: Math.round((parent.height - self.height) / 2 / 1px) * 1px;
             tint: #ffffff;
         }
     }
@@ -139,12 +139,12 @@ export component TrackPlayCell inherits Rectangle {
             text: root.number;
             color: Theme.text-muted;
             font-size: Typography.body;
-            x: (parent.width - self.width) / 2;
-            y: (parent.height - self.height) / 2;
+            x: Math.round((parent.width - self.width) / 2 / 1px) * 1px;
+            y: Math.round((parent.height - self.height) / 2 / 1px) * 1px;
         }
         if root.show-bars: EqualizerBars {
-            x: (parent.width - self.width) / 2;
-            y: (parent.height - self.height) / 2;
+            x: Math.round((parent.width - self.width) / 2 / 1px) * 1px;
+            y: Math.round((parent.height - self.height) / 2 / 1px) * 1px;
         }
         if root.show-overlay && !root.show-bars: QbzIcon {
             source: (root.is-playing && root.row-hovered)
@@ -152,8 +152,8 @@ export component TrackPlayCell inherits Rectangle {
                 : @image-url("../assets/icons/play-fill.svg");
             width: 16px;
             height: 16px;
-            x: (parent.width - self.width) / 2;
-            y: (parent.height - self.height) / 2;
+            x: Math.round((parent.width - self.width) / 2 / 1px) * 1px;
+            y: Math.round((parent.height - self.height) / 2 / 1px) * 1px;
             tint: #ffffff;
         }
     }