Fix shakapacker.yml template to prevent 'Slow setup' warnings in development (#1850)

The installer was generating shakapacker.yml with compile: true in development,
causing unnecessary 'Slow setup for development' warnings when using Procfiles
with bin/dev that already run bin/shakapacker-dev-server or bin/shakapacker --watch.

Changes:
- Set compile: false in default section (inherited by development and production)
- Remove compile: true from development section (inherits false from default)
- Keep compile: true in test section (on-demand compilation needed for tests)
- Add clarifying comments explaining when to use compile: true vs false

This prevents Rails from attempting on-demand compilation when a webpack dev
server is already running, eliminating confusing warnings on every page load.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: justin808

lib/generators/react_on_rails/templates/base/base/config/shakapacker.yml

@@ -55,6 +55,11 @@ default: &default
   # https://webpack.js.org/guides/build-performance/#avoid-production-specific-tooling
   useContentHash: false
 
+  # On-demand compilation of packs when modified. Defaults to false.
+  # Set to false if using bin/shakapacker-dev-server or bin/shakapacker --watch via Procfiles.
+  # Set to true only in test environment for on-demand compilation.
+  compile: false
+
   # Setting the asset host here will override Rails.application.config.asset_host.
   # Here, you can set different asset_host per environment. Note that
   # SHAKAPACKER_ASSET_HOST will override both configurations.
@@ -72,7 +77,6 @@ default: &default
 
 development:
   <<: *default
-  compile: true
   compiler_strategy: mtime
 
   # Reference: https://webpack.js.org/configuration/dev-server/