[PictureLoader] Fix worker thread shutdown and cross-thread cache clearing (#7292)

* [PictureLoader] Fix worker thread shutdown and cross-thread cache clearing

clearNetworkCache() ran directly on the UI thread while the worker thread
owned the disk cache and redirect cache, racing cache reads/writes. Make it
a worker-thread slot invoked via a blocking queued call when the thread is
running, so the 'Cached card pictures have been reset.' message is truthful.

The worker thread was also never quit()/wait()ed: both destructors only
deleteLater'd their objects, so Qt warned 'QThread: Destroyed while thread
is still running' and leaked a running loop at exit. Wire the worker's
finished() signal to its own deleteLater() (canonical worker-object
pattern), add shutdownThread() to stop the loop, and let CardPictureLoader
destroy the QThread only after wait() has returned.

* [PictureLoader] Guard cache teardown and stop blocking the UI thread

---------

Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>

cockatrice/src/interface/card_picture_loader/card_picture_loader.h

@@ -117,6 +117,9 @@ public:
 public slots:
     /**
      * @brief Clears the network disk cache of the worker.
+     *
+     * The clear runs on the worker thread, so this returns before it has completed; connect to
+     * networkCacheCleared() to act once it is done.
      */
     static void clearNetworkCache();
 
@@ -131,6 +134,10 @@ public slots:
     void installPrintingOverride(const ExactCard &originalCard, const ExactCard &overrideCard);
     void installPrintingOverrideOnLoad(const ExactCard &originalCard, const ExactCard &overrideCard);
 
+signals:
+    /** @brief Emitted after the worker has finished clearing the network and redirect caches. */
+    void networkCacheCleared();
+
 private slots:
     /**
      * @brief Triggered when the user changes the picture download settings.