chore(app): launcher icons, native splash, and web platform

Add flutter_launcher_icons + flutter_native_splash config (green sprout
on the #E7F1E0 canvas, light & dark) and regenerate Android/iOS assets.
Enable the web platform (favicon, manifest, PWA icons). Add
package_info_plus for the version string shown on the About screen.

apps/app_seeds/ios/Runner/Assets.xcassets/LaunchBackground.imageset/Contents.json

@@ -0,0 +1,22 @@
+{
+  "images" : [
+    {
+      "filename" : "background.png",
+      "idiom" : "universal"
+    },
+    {
+      "appearances" : [
+        {
+          "appearance" : "luminosity",
+          "value" : "dark"
+        }
+      ],
+      "filename" : "darkbackground.png",
+      "idiom" : "universal"
+    }
+  ],
+  "info" : {
+    "author" : "xcode",
+    "version" : 1
+  }
+}