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.

generator.go

@@ -713,6 +713,14 @@ func (g *Generator) copyStatic() error {
 		if err := os.WriteFile(dst, data, 0644); err != nil {
 			return err
 		}
+
+		// Also serve the favicon from the site root, where browsers and
+		// crawlers request /favicon.ico by default without a <link> tag.
+		if e.Name() == "favicon.ico" {
+			if err := os.WriteFile(filepath.Join(g.outputDir, e.Name()), data, 0644); err != nil {
+				return err
+			}
+		}
 	}
 	return nil
 }