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.

templates/grid.html

@@ -6,6 +6,9 @@
   <title>Photo Gallery</title>
   <link rel="stylesheet" href="{{.FontsURL}}">
   <link rel="stylesheet" href="/static/style.css">
+  <link rel="icon" href="/static/favicon.ico" sizes="any">
+  <link rel="icon" type="image/png" href="/static/web-32.png" sizes="32x32">
+  <link rel="apple-touch-icon" href="/static/web-192.png">
   <style>:root { --serif: '{{.SerifFamily}}', Georgia, serif; --mj: '{{.MlFamily}}', sans-serif; }</style>
   <link rel="alternate" type="application/atom+xml" href="/feed.xml" title="Photo Feed">
 </head>