Increase flash message display timeout to 8 seconds

The success/error banners (e.g. "Check your email for a link") were
dismissing after only 3 seconds, too quick to read comfortably.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

public/web.config

@@ -14,7 +14,7 @@
     <add key="Environment" value="Development" />
 
     <!-- Flash message display duration in milliseconds -->
-    <add key="FlashMessageTimeout" value="3000" />
+    <add key="FlashMessageTimeout" value="8000" />
 
     <!-- 404 error page redirect countdown in seconds -->
     <add key="Error404RedirectSeconds" value="5" />

public/web.config.production.template

@@ -12,7 +12,7 @@
     <add key="Environment" value="Production" />
 
     <!-- Flash message display duration in milliseconds -->
-    <add key="FlashMessageTimeout" value="3000" />
+    <add key="FlashMessageTimeout" value="8000" />
 
     <!-- 404 error page redirect countdown in seconds -->
     <add key="Error404RedirectSeconds" value="5" />