refactor: split MainWindow constructor into focused setup methods

Extract the ~300-line constructor body into setupDocks(), setupScrobbler(),
setupGapless(), setupMpris(), connectBackendSignals(), connectLibrarySignals(),
connectContentSignals(), and connectToolbarSignals(). No behavioral changes;
all signal/slot connections and widget creation remain identical.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/mainwindow.hpp

@@ -72,5 +72,13 @@ private:
     bool m_nextTrackPrefetched = false;
 
     void setupMenuBar();
+    void setupDocks();
+    void setupScrobbler();
+    void setupGapless();
+    void setupMpris();
+    void connectBackendSignals();
+    void connectLibrarySignals();
+    void connectContentSignals();
+    void connectToolbarSignals();
     void tryRestoreSession();
 };