feat: shared modal system + migrate tasks module

- Add public/components/modal.js with focus-trap, escape-handler,
  overlay-click, focus-restore, scroll-lock, aria-modal (Spec §5.1/§5.2)
- Migrate tasks.js from custom modal to shared openModal/closeModal API
- Remove .modal-backdrop/.modal/.modal__* styles from tasks.css
- Add .modal-panel--sm/--lg sizing variants to layout.css

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

public/styles/layout.css

@@ -599,6 +599,8 @@
     border-radius: var(--radius-lg);
     animation: modal-scale-in 0.2s var(--ease-out) forwards;
   }
+  .modal-panel--sm { max-width: 400px; }
+  .modal-panel--lg { max-width: 680px; }
 }
 
 .modal-panel__header {