Fall back to localhost for mailer host when HOST is unset

ENV.fetch("HOST") at config-load time crashed any Rails invocation
that doesn't go through Foreman (bin/rails test, console, setup, CI)
— .env is loaded only when bin/dev runs via Foreman/Overmind. Default
the mailer URL host to "localhost" so boot succeeds in those paths;
Procfile.dev still uses \${HOST:?...} so the dev server itself won't
start without an explicit Wi-Fi IP.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: dadachi

config/environments/development.rb

@@ -38,7 +38,11 @@
 
   config.action_mailer.perform_caching = false
 
-  config.action_mailer.default_url_options = {host: ENV.fetch("HOST"), port: ENV.fetch("PORT", 3000).to_i}
+  # HOST is only loaded into ENV by Foreman under `bin/dev`; direct Rails
+  # invocations (console, test, setup, CI) don't see .env, so default to
+  # localhost for the mailer so boot succeeds. `Procfile.dev` still refuses
+  # to start the dev server without an explicit HOST.
+  config.action_mailer.default_url_options = {host: ENV.fetch("HOST", "localhost"), port: ENV.fetch("PORT", 3000).to_i}
 
   # Print deprecation notices to the Rails logger.
   config.active_support.deprecation = :log