feat(gui): enhance app icons with multi-resolution support

Improves the application's icon handling by introducing multi-resolution
support, replacing a single PNG file with multiple resolution-specific icons.

- ✨ **Added** multiple resolution icons (16x16, 32x32, 48x48, 64x64, 256x256, 512x512).
- 🔄 **Updated** GUI setup to use multi-resolution icons for better scalability.
- 🗑️ **Removed** the old single-resolution `icon.png`.

This update ensures better compatibility and visual quality across
different platforms and screen resolutions.

tidal_dl_ng/gui.py

@@ -5,7 +5,7 @@
 #    nuitka-project: --macos-signed-app-name=com.exislow.TidalDlNg
 #    nuitka-project: --macos-app-mode=gui
 # nuitka-project-if: {OS} in ("Linux", "FreeBSD"):
-#    nuitka-project: --linux-icon=tidal_dl_ng/ui/icon.png
+#    nuitka-project: --linux-icon=tidal_dl_ng/ui/icon512.png
 # nuitka-project-if: {OS} in ("Windows"):
 #    nuitka-project: --windows-icon-from-ico=tidal_dl_ng/ui/icon.ico
 #    nuitka-project: --file-description="TIDAL media downloader next generation."
@@ -1692,9 +1692,14 @@ def gui_activate(tidal: Tidal | None = None):
     qdarktheme.setup_theme(additional_qss="QToolTip { border: 0px; }")
 
     # Create icon object and apply it to app window.
-    pixmap: QtGui.QPixmap = QtGui.QPixmap("tidal_dl_ng/ui/icon.png")
-    icon: QtGui.QIcon = QtGui.QIcon(pixmap)
+    icon: QtGui.QIcon = QtGui.QIcon()
 
+    icon.addFile("tidal_dl_ng/ui/icon16.png", QtCore.QSize(16, 16))
+    icon.addFile("tidal_dl_ng/ui/icon32.png", QtCore.QSize(32, 32))
+    icon.addFile("tidal_dl_ng/ui/icon48.png", QtCore.QSize(48, 48))
+    icon.addFile("tidal_dl_ng/ui/icon64.png", QtCore.QSize(64, 64))
+    icon.addFile("tidal_dl_ng/ui/icon256.png", QtCore.QSize(256, 256))
+    icon.addFile("tidal_dl_ng/ui/icon512.png", QtCore.QSize(512, 512))
     app.setWindowIcon(icon)
 
     # This bit gets the taskbar icon working properly in Windows