feat(qt): dim the unsung part of the immersive karaoke line

LyricsLineRow gains inactiveColor (default theme.textSecondary, so the
desktop lyrics panel is unchanged) and LyricsLinesView forwards it. The
immersive lyric surfaces pass 35% white: their active overlay is solid
white and the theme's textSecondary base was barely dimmer, so the timed
karaoke fill read as static text.

cc

crates/qbz-qt/qml/shell/LyricsLineRow.qml

@@ -54,6 +54,11 @@ Column {
     property bool uppercase: false
     property string fontFamily: ""
     property color activeColor: theme.accent
+    // The UNSUNG part of the active karaoke line. The default follows the
+    // theme; immersive hosts override it with a decisively dimmer white —
+    // theme.textSecondary is near-white there, so the timed fill was barely
+    // distinguishable from the sung overlay (2026-08-31 visual cleanup).
+    property color inactiveColor: theme.textSecondary
     // Lite fill: light the active line whole, no per-frame clip sweep.
     property bool liteFill: false
     property bool showTranslation: false
@@ -216,7 +221,7 @@ Column {
                     id: segBase
                     text: seg.modelData.text
                     wrapMode: Text.NoWrap
-                    color: theme.textSecondary
+                    color: row.inactiveColor
                     font.family: row.effFamily
                     font.pixelSize: row.sizeActive
                     font.weight: theme.weightBold

crates/qbz-qt/qml/shell/LyricsLinesView.qml

@@ -51,6 +51,8 @@ Item {
     // 0 System · 1 LINE Seed JP · 2 Montserrat · 3 Noto Sans · 4 Source Sans 3
     property int fontIndex: 0
     property color activeColor: theme.accent
+    // Unsung part of the active karaoke line (see LyricsLineRow.qml).
+    property color inactiveColor: theme.textSecondary
     property bool liteFill: false
 
     // Seek request from a line click (the host decides whether to honour it).
@@ -194,6 +196,7 @@ Item {
                         uppercase: view.uppercase
                         fontFamily: view.fontFamily
                         activeColor: view.activeColor
+                        inactiveColor: view.inactiveColor
                         liteFill: view.liteFill
                         showTranslation: view.showTranslation
                     }