feat(slint): per-row offline affordance + the unlock padlock (Slice 2 UI)

TrackRow now renders the offline state the controller maintains:

- The offline button is stateful: cloud-download (none, click -> cache) ->
  loader-circle accent (queued/downloading) -> circle-check accent (ready,
  click -> uncache) -> triangle-alert red (failed, click -> retry). Shown on
  surfaces that enable it AND on any row with active cache state.
- The unlock PADLOCK (user-requested, back from Tauri): a pulsing lock glyph
  overlays the play cell while the track's CMAF bundle decrypts, driven by the
  `unlocking` flag the row_sink toggles on UnlockStart/UnlockEnd. New lucide
  lock.svg / lock-open.svg assets.

The context menu's "Make available offline" already routes to caching. Testable
loop: cache a track (album button or context menu) -> watch it download -> play
it offline -> padlock pulses during decrypt. qbz-slint compiles clean.

cc

crates/qbz-slint/ui/assets/icons/lock-open.svg

@@ -0,0 +1,4 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="#ffffff" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+  <rect width="18" height="11" x="3" y="11" rx="2" ry="2" />
+  <path d="M7 11V7a5 5 0 0 1 9.9-1" />
+</svg>

crates/qbz-slint/ui/assets/icons/lock.svg

@@ -0,0 +1,4 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="#ffffff" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+  <rect width="18" height="11" x="3" y="11" rx="2" ry="2" />
+  <path d="M7 11V7a5 5 0 0 1 10 0v4" />
+</svg>

crates/qbz-slint/ui/primitives/TrackRow.slint

@@ -220,6 +220,34 @@ export component TrackRow inherits Rectangle {
                         }
                     }
                 }
+
+                // Offline-decrypt padlock — overlays the play cell while the
+                // track's CMAF bundle is being decrypted for playback. Pulses
+                // to read as "unlocking" (mirrors Tauri's unlock glyph).
+                if root.track.unlocking: lock-overlay := Rectangle {
+                    property <bool> pulse;
+                    width: parent.width;
+                    height: parent.height;
+                    border-radius: 6px;
+                    background: #000000aa;
+                    QbzIcon {
+                        source: @image-url("../assets/icons/lock.svg");
+                        width: 15px;
+                        height: 15px;
+                        x: (parent.width - self.width) / 2;
+                        y: (parent.height - self.height) / 2;
+                        tint: Theme.accent;
+                        opacity: lock-overlay.pulse ? 1.0 : 0.4;
+                        animate opacity { duration: 450ms; }
+                    }
+                    Timer {
+                        interval: 450ms;
+                        running: true;
+                        triggered => {
+                            lock-overlay.pulse = !lock-overlay.pulse;
+                        }
+                    }
+                }
             }
         }
 
@@ -365,8 +393,15 @@ export component TrackRow inherits Rectangle {
                 }
             }
         }
-        // Offline cache.
+        // Offline cache — a stateful affordance reflecting cache-status:
-        if root.show-download: VerticalLayout {
+        //   0 none        cloud-download  (click -> cache)
+        //   1/2 q'd/dl'ing loader (accent) (click -> cache; re-queues)
+        //   3 ready       circle-check    (click -> uncache)
+        //   4 failed      triangle-alert  (click -> cache; retries)
+        // Shown when the surface enables it OR the track has active state, so
+        // a downloading/ready copy surfaces its status even where the button
+        // isn't enabled by default.
+        if root.show-download || root.track.cache-status > 0: VerticalLayout {
             alignment: center;
             Rectangle {
                 width: 28px;
@@ -374,17 +409,30 @@ export component TrackRow inherits Rectangle {
                 border-radius: Radius.sm;
                 background: dl-ta.has-hover ? Theme.surface-elevated : transparent;
                 QbzIcon {
-                    source: @image-url("../assets/icons/cloud-download.svg");
+                    source: root.track.cache-status == 3
+                        ? @image-url("../assets/icons/circle-check-big.svg")
+                        : (root.track.cache-status == 4
+                            ? @image-url("../assets/icons/triangle-alert.svg")
+                            : ((root.track.cache-status == 1 || root.track.cache-status == 2)
+                                ? @image-url("../assets/icons/loader-circle.svg")
+                                : @image-url("../assets/icons/cloud-download.svg")));
                     width: 16px;
                     height: 16px;
                     x: (parent.width - self.width) / 2;
                     y: (parent.height - self.height) / 2;
-                    tint: dl-ta.has-hover ? Theme.text-primary : Theme.text-muted;
+                    tint: root.track.cache-status == 3
+                        ? Theme.accent
+                        : (root.track.cache-status == 4
+                            ? #e0564f
+                            : ((root.track.cache-status == 1 || root.track.cache-status == 2)
+                                ? Theme.accent
+                                : (dl-ta.has-hover ? Theme.text-primary : Theme.text-muted)));
                 }
                 dl-ta := TouchArea {
                     mouse-cursor: pointer;
                     clicked => {
-                        root.media-action("track", root.track.id, "download");
+                        root.media-action("track", root.track.id,
+                            root.track.cache-status == 3 ? "uncache" : "cache");
                     }
                 }
             }