feat(ui): carry lyrics translation state and render bilingual lines

LyricsLineItem gains translation-text (empty when the doc has none) and
LyricsState the translation props (translation-available, show-translation,
toggle-translation callback) per spec section C. LyricsLinesView renders
the translation under its original line while show-translation is on —
smaller (~72%) and dimmer, static text that follows the line; the karaoke
fill stays keyed on the original timings/text. Secondary surfaces share
the model, so the field flows through untouched.

km

crates/qbz-ui/ui/shell/LyricsLinesView.slint

@@ -70,6 +70,9 @@ import "../../../../static/fonts/SourceSans3-VariableFont_wght.ttf";
 component LyricsLineRow inherits VerticalLayout {
     in property <int> index;
     in property <string> line-text;
+    // Translation of this line (Qobuz v10); empty when the doc has none.
+    // Rendered under the original line while LyricsState.show-translation.
+    in property <string> translation-text;
     in property <length> size-inactive;
     in property <length> size-active;
     // 0 = off, 1 = soft, 2 = strong (Tauri default) — pref-driven (S5).
@@ -86,6 +89,10 @@ component LyricsLineRow inherits VerticalLayout {
     // eager init already saw it active.
     in property <int> recenter-epoch;
 
+    // Gap between the original line and its translation (no effect while
+    // the translation is hidden — a single child has no spacing).
+    spacing: 3px;
+
     // Tauri renders uppercase via CSS text-transform; Slint transforms the
     // string (display only — the engine still matches on the raw text).
     property <string> display-text:
@@ -255,6 +262,26 @@ component LyricsLineRow inherits VerticalLayout {
             }
         }
     }
+
+    // Translation (Qobuz v10): static text UNDER the original line — smaller
+    // (~72%, mobile reference) and dimmer. The karaoke fill above stays keyed
+    // on the ORIGINAL timings/text; this follows its line statically and
+    // joins the same dimming ladder (never uppercased — it's a translation,
+    // not the lyric). No-op while the toggle is off or the doc has none.
+    if LyricsState.show-translation && root.translation-text != "": Text {
+        text: root.translation-text;
+        wrap: word-wrap;
+        opacity: root.line-opacity();
+        color: root.is-past ? Theme.text-muted : Theme.text-secondary.with-alpha(0.7);
+        font-family: root.font-name;
+        font-size: (root.is-active ? root.size-active : root.size-inactive) * 0.72;
+        font-weight: Typography.regular;
+        letter-spacing: 0.2px;
+        animate opacity {
+            duration: 220ms;
+            easing: ease-out;
+        }
+    }
 }
 
 export component LyricsLinesView inherits Rectangle {
@@ -386,6 +413,7 @@ export component LyricsLinesView inherits Rectangle {
             for line[idx] in LyricsState.lines: LyricsLineRow {
                 index: idx;
                 line-text: line.text;
+                translation-text: line.translation-text;
                 size-inactive: root.size-inactive;
                 size-active: root.size-active;
                 dimming-mode: root.dimming-mode;

crates/qbz-ui/ui/state.slint

@@ -4454,6 +4454,11 @@ export struct LyricsLineItem {
     time-ms: int,
     end-ms: int,
     has-words: bool,
+    // Translation of this line (Qobuz v10), aligned 1:1 with `text` by the
+    // Rust glue; empty when the doc has no translation (or it wasn't
+    // requested). Rendered under the original line only while
+    // `LyricsState.show-translation` is on.
+    translation-text: string,
 }
 
 // One VISUAL line of the ACTIVE logical line, for the per-visual-line karaoke
@@ -4488,6 +4493,18 @@ export global LyricsState {
     // Playing-track meta for the header ("{title} - {artist}").
     in property <string> track-title;
     in property <string> track-artist;
+
+    // ---- Translation (Qobuz API v10) ---------------------------------------
+    // The current doc advertises at least one translation language
+    // (`translation_langs` non-empty) — gates the sidebar's floating toggle.
+    // False on every other state, so the sidebar renders exactly as before.
+    in property <bool> translation-available: false;
+    // User intent: render the translation under each original line. Driven by
+    // Rust (`toggle-translation`): ON refetches with the target language and
+    // flips this only when the translation actually arrived; OFF is pure UI.
+    in-out property <bool> show-translation: false;
+    // Sidebar floating-button action (wired in main.rs).
+    callback toggle-translation();
     // Surfaced backend error text (status == 4) — dynamic content, not chrome.
     in property <string> error;
     // Active line index; -1 = none. The S4 sync engine drives it.