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/app.qrc

@@ -0,0 +1,9 @@
+<!DOCTYPE RCC>
+<RCC version="1.0">
+    <qresource prefix="/">
+        <file>icons/app-32.png</file>
+        <file>icons/app-128.png</file>
+        <file>icons/app-256.png</file>
+        <file>icons/app-512.png</file>
+    </qresource>
+</RCC>