[dashboard] Add apple-app-site-association for iOS deep link testing (#4891)

This PR adds the necessary configuration to support iOS Universal Links verification on the dashboard domain.

Changes:

Adds web/.well-known/apple-app-site-association containing the App IDs for the Flutter iOS team's test apps.

Updates firebase.json to ensure the AASA file is served with the required Content-Type: application/json header.

Reason: This setup allows Apple to verify the flutter-dashboard domain, enabling the team to test deep linking workflows on iOS devices.

*List which issues are fixed by this PR. You must list at least one issue.*

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*
diff --git a/dashboard/web/.well-known/apple-app-site-association b/dashboard/web/.well-known/apple-app-site-association
new file mode 100644
index 0000000..8d2266a
--- /dev/null
+++ b/dashboard/web/.well-known/apple-app-site-association
@@ -0,0 +1,14 @@
+{
+  "applinks": {
+    "apps": [],
+    "details": [
+      {
+        "appIDs": ["EQHXZ8M8AV.com.google.experimental0.dev"],
+        "paths": ["*"]
+      }
+    ]
+  },
+  "webcredentials": {
+    "apps": ["EQHXZ8M8AV.com.google.experimental0.dev"]
+  }
+}