Add app icons across web, Android, and the desktop app

- Web: favicon.ico + png icons under static/, served from the site root
  and linked from every page head.
- Android: the artwork becomes the launcher icon — the 432px adaptive
  foreground over the gruvbox background, replacing the placeholder vector
  drawables.
- Desktop: the icon is the Qt window/taskbar icon (embedded via app.qrc),
  and the app is now an installable package — a wrapped binary, a
  photog.desktop entry, and hicolor icons — so `nix profile install
  ./desktop` puts "Photog" in the application menu.

desktop/main.cpp

@@ -1,4 +1,5 @@
 #include <QApplication>
+#include <QIcon>
 
 #include "mainwindow.h"
 
@@ -8,6 +9,15 @@ int main(int argc, char **argv)
     app.setApplicationName(QStringLiteral("photogallery-uploader"));
     app.setOrganizationName(QStringLiteral("inflo.ws"));
 
+    // Window / taskbar / alt-tab icon, embedded via app.qrc. Several sizes so
+    // the small titlebar rendering stays crisp rather than downscaling 512px.
+    QIcon icon;
+    icon.addFile(QStringLiteral(":/icons/app-32.png"));
+    icon.addFile(QStringLiteral(":/icons/app-128.png"));
+    icon.addFile(QStringLiteral(":/icons/app-256.png"));
+    icon.addFile(QStringLiteral(":/icons/app-512.png"));
+    app.setWindowIcon(icon);
+
     MainWindow window;
     window.resize(440, 680);
     window.show();