feat: update sharing image url and rules

storage.rules

@@ -0,0 +1,9 @@
+rules_version = '2';
+service firebase.storage {
+  match /b/{bucket}/o {
+    match /{folder}/{imageId} {
+    	allow read: if imageId.matches(".*\\.png") || imageId.matches(".*\\.jpg");
+      allow write: if false;
+    }
+  }
+}

web/index.html

@@ -30,10 +30,10 @@
   <meta property="og:url" content="https://flutter.dev">
 
   <meta name="twitter:image"
-    content="https://firebasestorage.googleapis.com/v0/b/pinball-dev.appspot.com/o/pinball_metadata_image.png?alt=media&token=071d8892-3328-4447-804a-147b46656f24">
+    content="https://firebasestorage.googleapis.com/v0/b/pinball-dev.appspot.com/o/images%2Fpinball_share_image.png?alt=media">
 
   <meta property="og:image"
-    content="https://firebasestorage.googleapis.com/v0/b/pinball-dev.appspot.com/o/pinball_metadata_image.png?alt=media&token=071d8892-3328-4447-804a-147b46656f24">
+    content="https://firebasestorage.googleapis.com/v0/b/pinball-dev.appspot.com/o/images%2Fpinball_share_image.png?alt=media">
 
   <meta name="twitter:text:title" content="I/O Pinball Machine - Flutter">
   <meta name="twitter:card" content="summary_large_image">