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/JumpNavBar.slint

@@ -224,8 +224,8 @@ export component JumpNavBar inherits Rectangle {
                                 source: @image-url("../assets/icons/x.svg");
                                 width: 12px;
                                 height: 12px;
-                                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: clear-ta.has-hover
                                     ? Theme.text-primary
                                     : Theme.text-muted;
@@ -262,8 +262,8 @@ export component JumpNavBar inherits Rectangle {
                     source: @image-url("../assets/icons/search.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: search-ta.has-hover
                         ? Theme.text-primary
                         : Theme.text-muted;