Default development environment to app.fizzy.localhost (#2822)

Using fizzy.localhost causes CORS errors when using minio for Active
Storage because the minio endpoint is at minio.fizzy.localhost — a
sibling subdomain, not a subdomain of the app host. Switching to
app.fizzy.localhost makes both hosts subdomains of fizzy.localhost,
resolving the CORS issue. See #2814 for the related minio CORS fix.

fizzy.localhost will continue to work if people want to use it, but all
docs and scripts have been updated to point to app.fizzy.localhost.

docs/api/sections/notifications.md

@@ -24,15 +24,15 @@ __Response:__
       "active": true,
       "email_address": "david@example.com",
       "created_at": "2025-12-05T19:36:35.401Z",
-      "url": "http://fizzy.localhost:3006/897362094/users/03f5v9zjw7pz8717a4no1h8a7"
+      "url": "http://app.fizzy.localhost:3006/897362094/users/03f5v9zjw7pz8717a4no1h8a7"
     },
     "card": {
       "id": "03f5v9zo9qlcwwpyc0ascnikz",
       "title": "Plain text mentions",
       "status": "published",
-      "url": "http://fizzy.localhost:3006/897362094/cards/3"
+      "url": "http://app.fizzy.localhost:3006/897362094/cards/3"
     },
-    "url": "http://fizzy.localhost:3006/897362094/notifications/03f5va03bpuvkcjemcxl73ho2"
+    "url": "http://app.fizzy.localhost:3006/897362094/notifications/03f5va03bpuvkcjemcxl73ho2"
   }
 ]
 ```