feat(slint): NowPlayingBar rebuild, Settings constraints, Discover fixes

NowPlayingBar (start of the playback-bar work):
- height raised to 110px
- SeekBar component: three-line track (total / cached / progress), a
  hover thumb, elapsed/remaining times, and a hover time bubble with a
  downward caret marking the seek position
- song card: background and border dropped, gains the extra height,
  album/artist links and the context icon kept (context icon now
  always shown)
- transport + secondary buttons copied from Tauri (shuffle, prev,
  play, next, repeat, add, favorite; cast, picture-in-picture,
  fullscreen, normalization; volume cluster). Queue/Lyrics stay in the
  header — and the header now orders them lyrics-then-queue.

Settings constraints (1:1 with Tauri):
- Exclusive Mode enabled only on ALSA; DAC Passthrough only on
  PipeWire; Lock Output Device disabled while DAC Passthrough is on;
  Gapless disabled while Streaming Only is on
- backend change cascades: forces off DAC passthrough / force-bit-
  perfect / exclusive / gapless as appropriate; DAC-passthrough and
  streaming-only cascades; initial-buffer slider right-aligned

Discover fixes:
- navigation [<] [>] pair homologated to the same top/left margin
  across every view
- album/artist description moved back into the metadata column (it now
  fills the width there) with uniform secondary text color
- genre filter button hugs its label instead of stretching
- Recently Played Albums and Tracks modules always render, with a
  placeholder until the play history has data
- wider spacing between Discover rows; the Discover gear is no longer
  a Settings shortcut (it is the per-tab sort/show-hide control)

cc

crates/qbz-slint/ui/album/AlbumPageView.slint

@@ -314,9 +314,11 @@ export component AlbumPageView inherits Rectangle {
 
         page := VerticalLayout {
             width: parent.width;
-            padding-left: 24px;
-            padding-right: 24px;
-            padding-top: 14px;
+            // padding matches Discover so the [<] [>] pair sits at the same
+            // top/left margin across every view.
+            padding-left: 32px;
+            padding-right: 32px;
+            padding-top: 11px;
             padding-bottom: 100px;
             spacing: 0px;
             alignment: start;
@@ -386,11 +388,39 @@ export component AlbumPageView inherits Rectangle {
                     Rectangle { height: 10px; }
                     Text {
                         text: AlbumState.info-line;
-                        color: Theme.text-muted;
+                        color: Theme.text-secondary;
                         font-size: Typography.body;
                         overflow: elide;
                     }
 
+                    // Editorial description — fills the metadata column's
+                    // width (the dead space right of the artwork), truncated,
+                    // with a Read more link to the full modal.
+                    if AlbumState.description != "": Rectangle { height: 12px; }
+                    if AlbumState.description != "": Text {
+                        text: AlbumState.description-short;
+                        color: Theme.text-secondary;
+                        font-size: Typography.legal;
+                        wrap: word-wrap;
+                    }
+                    if AlbumState.description != AlbumState.description-short: Rectangle { height: 4px; }
+                    if AlbumState.description != AlbumState.description-short: read-more := TouchArea {
+                        width: read-more-row.preferred-width;
+                        height: 20px;
+                        mouse-cursor: pointer;
+                        clicked => {
+                            root.show-description = true;
+                        }
+                        read-more-row := HorizontalLayout {
+                            Text {
+                                text: @tr("Read more");
+                                color: read-more.has-hover ? Theme.accent-hover : Theme.accent;
+                                font-size: Typography.legal;
+                                vertical-alignment: center;
+                            }
+                        }
+                    }
+
                     Rectangle { height: 20px; }
                     HorizontalLayout {
                         alignment: start;
@@ -444,33 +474,6 @@ export component AlbumPageView inherits Rectangle {
                 }
             }
 
-            // Editorial description — full content width, pre-truncated so
-            // it wraps cleanly, with a Read more link to the full modal.
-            if AlbumState.description != "": Rectangle { height: 18px; }
-            if AlbumState.description != "": Text {
-                text: AlbumState.description-short;
-                color: Theme.text-secondary;
-                font-size: Typography.body;
-                wrap: word-wrap;
-            }
-            if AlbumState.description != AlbumState.description-short: Rectangle { height: 4px; }
-            if AlbumState.description != AlbumState.description-short: read-more := TouchArea {
-                width: read-more-row.preferred-width;
-                height: 22px;
-                mouse-cursor: pointer;
-                clicked => {
-                    root.show-description = true;
-                }
-                read-more-row := HorizontalLayout {
-                    Text {
-                        text: @tr("Read more");
-                        color: read-more.has-hover ? Theme.accent-hover : Theme.accent;
-                        font-size: Typography.legal;
-                        vertical-alignment: center;
-                    }
-                }
-            }
-
             Rectangle { height: 20px; }
             Rectangle {
                 height: 1px;