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/artist/ArtistPageView.slint

@@ -193,9 +193,11 @@ export component ArtistPageView 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;
@@ -251,6 +253,14 @@ export component ArtistPageView inherits Rectangle {
                         font-weight: Typography.bold;
                         overflow: elide;
                     }
+                    Rectangle { height: 12px; }
+                    Text {
+                        text: ArtistState.bio;
+                        visible: ArtistState.bio != "";
+                        color: Theme.text-secondary;
+                        font-size: Typography.body;
+                        wrap: word-wrap;
+                    }
                     Rectangle { height: 18px; }
                     HorizontalLayout {
                         alignment: start;
@@ -280,15 +290,6 @@ export component ArtistPageView inherits Rectangle {
                 }
             }
 
-            // Biography — full content width, below the header block.
-            if ArtistState.bio != "": Rectangle { height: 20px; }
-            if ArtistState.bio != "": Text {
-                text: ArtistState.bio;
-                color: Theme.text-secondary;
-                font-size: Typography.body;
-                wrap: word-wrap;
-            }
-
             Rectangle { height: 36px; }
 
             if ArtistState.loading && ArtistState.top-tracks.length == 0: Text {