Toast Stacking: Container Controller, Templates, and Integration

What Was Built

• New notification_container_controller.js Stimulus controller: MutationObserver-based toast overflow capping (max 3 visible), expand/collapse with click-outside, "+N more" badge, "Clear all" action, debounced recalculation via requestAnimationFrame, accessibility (aria-hidden + inert on hidden toasts)
• Modified notification_controller.js: dispatches notification:dismissed custom event before fade-out, error-level toasts get 10s auto-dismiss (vs 5s default)
• Layout template wired with data-controller="notification-container", list target, badge with count, and clear-all button
• Toast partial includes data-level attribute for error delay detection
• Controller registered in application.js

Files Modified

• app/assets/javascripts/source_monitor/controllers/notification_container_controller.js (CREATE, 131 lines)
• app/assets/javascripts/source_monitor/controllers/notification_controller.js (MODIFY, +11 lines)
• app/views/layouts/source_monitor/application.html.erb (MODIFY, +22/-2 lines)
• app/views/source_monitor/shared/_toast.html.erb (MODIFY, +1 line)
• app/assets/javascripts/source_monitor/application.js (MODIFY, +2 lines)