fix(qt): saving the queue as a playlist works with local rows, and offline

queue_qt::save_as_playlist skipped every local and ephemeral row and,
with no catalog ids left, returned without a word — a queue of Local
Library tracks opened nothing from either the queue panel or the Listen
list, online or offline. The module's own note said the picker's local
mode existed and only the row-to-ref resolver was missing.

local_playlist_qt::local_picker_ref_for_queue_track now builds that ref
from a QueueTrack by source (a file its row id, Plex its rating key,
Jellyfin/Subsonic the server item id, an offline copy its row id from
the hint slot; an ephemeral row nothing), and the queue carries ids and
refs together: only local rows open the picker in local mode, a mix
carries the new Payload::Mixed, which the picker routes as the two halves
would be on their own — both writes onto a local playlist, the sidecar
plus the API (duplicate check included) onto a Qobuz one, a local
playlist on create while offline. The per-row "Add to playlist" entry
shows for local rows on both queue surfaces too.

cc

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

@@ -319,9 +319,10 @@ Rectangle {
             items.push({ "label": t("Go to artist", r), "icon": "user",
                          "action": "go-artist", "external": true })
         if (row.isEphemeral !== true) {
-            if (row.isLocal !== true)
-                items.push({ "label": t("Add to playlist", r), "icon": "list-plus",
-                             "action": "add-to-playlist", "external": true })
+            // Local rows too (2026-09-14): the queue resolves them to picker
+            // refs by source (queue_qt::picker_payload_for).
+            items.push({ "label": t("Add to playlist", r), "icon": "list-plus",
+                         "action": "add-to-playlist", "external": true })
             items.push({ "label": t("Track info", r), "icon": "info", "action": "track-info" })
             if (row.isLocal !== true)
                 items.push({ "label": row.isFavorite === true