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/flake.nix

@@ -17,6 +17,28 @@
           pkgs.qt6.qtimageformats
         ];
       in {
+        # Installable build: `nix profile install ./desktop` drops "Photog" into
+        # the application menu with its icon. wrapQtAppsHook wraps the binary so
+        # it finds the Qt platform (xcb + wayland) and image-format plugins on
+        # its own, without the dev shell.
+        packages.default = pkgs.stdenv.mkDerivation {
+          pname = "photog";
+          version = "0.1.0";
+          src = ./.;
+          nativeBuildInputs = with pkgs; [
+            cmake
+            ninja
+            pkg-config
+            qt6.wrapQtAppsHook
+          ];
+          buildInputs = with pkgs; [
+            qt6.qtbase
+            qt6.qtwayland
+            qt6.qtimageformats
+            qt6Packages.qtkeychain
+          ];
+        };
+
         devShells.default = pkgs.mkShell {
           packages = with pkgs; [
             cmake