Tailscale serve support (#2126)

Ensure we can serve the app from multiple hosts without breaking links.
* Switch unnecessary full URLs to paths
* Drop default host/port URL options for controllers

Shell 1
```bash
bin/dev
```

Shell 2
```bash
tailscale serve http://fizzy.localhost:3006
```

config/environments/development.rb

@@ -85,9 +85,14 @@ Rails.application.configure do
     config.action_mailer.raise_delivery_errors = false
   end
 
-  config.hosts = %w[ fizzy.localhost localhost 127.0.0.1 ] + [ /^fizzy-\d+(:\d+)$/ ]
+  config.hosts = [
+    "fizzy.localhost",
+    "localhost",
+    "127.0.0.1",
+    /^fizzy-\d+(:\d+)?$/, # review apps: fizzy-123:3000
+    /\.ts\.net$/         # tailscale serve: hostname.tail1234.ts.net
+  ]
 
-  # Set host to be used by links generated in mailer and notification view templates.
-  config.action_controller.default_url_options = { host: config.hosts.first, port: 3006 }
-  config.action_mailer.default_url_options     = { host: config.hosts.first, port: 3006 }
+  # Canonical host for mailer URLs (emails always link here, not personal Tailscale URLs)
+  config.action_mailer.default_url_options = { host: "#{config.hosts.first}:3006" }
 end