feat(qt): the artist page's radio disc goes live, with a start-up indicator

The disc was btnEnabled: false with a comment saying neither engine had a seam
on a Qt bridge. Both do now. ArtistPageView.slint:426-461 opens a two-row
dropdown — QBZ Radio (the local qbz-radio pool builder) and Qobuz Radio
(/radio/artist) — and that is what this restores. The smart arm already
shipped behind Discover's spotlight RADIO card and simply was not wired here;
the Qobuz arm is new (get_radio_artist + the same enrich_radio_tracks the album
radio uses, because its track objects are just as bare).

Both labels already exist in all eight catalogues, so no new msgid.

AND THE WAIT IS VISIBLE NOW. A radio is a fetch, an enrich and a queue write;
until the first track resolves nothing on screen moves, so the click read as
dead. QbzCircleAction gains the `loading` arm the reference already has
(CircleAction.slint:18/:77) and QbzHome.radioPending carries the KEY of the
radio being built — "album:<id>" or "artist:<id>" — so the Discover rail spins
only the station that was clicked instead of all six.

The spinner rides QbzShell.pulseMs, NOT a RotationAnimation. Qt Quick has no
partial redraws, so a continuous animator presents the whole window at display
rate (~1.2% GPU per present on the owner's hybrid laptop); a 60 Hz spin for the
seconds a radio takes is exactly the bill the pulse law refuses. 12 degrees per
~30 Hz tick, and the Connections is disabled unless the button is spinning and
on screen, so at rest it writes nothing. theme/QbzSpinner.qml still uses a
RotationAnimation and is deliberately not reused.

On the cards, `loading` also pins the hover disc up: it only exists on hover,
so it vanished the moment the pointer left after a click, which is what made
the click look swallowed.

A RadioGuard disarms the key in Drop, so a bad id, a network error and an empty
result all clear it. A disc spinning forever is worse than no indicator.

cc

crates/qbz-qt/qml/views/AlbumView.qml

@@ -860,6 +860,11 @@ Rectangle {
                         QbzCircleAction {
                             name: "radio"
                             overlay: root.hdrOverlay
+                            // A radio is a fetch + an enrich + a queue write,
+                            // and until the first track resolves nothing on
+                            // screen moves. The key is per-album so this disc
+                            // spins and the Discover rail's stations do not.
+                            loading: QbzHome.radioPending === "album:" + albumHeader.id
                             anchors.verticalCenter: parent.verticalCenter
                             onClicked: QbzHome.startAlbumRadio(albumHeader.id)
                         }