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 {

public/styles/tasks.css

@@ -472,89 +472,6 @@
   color: var(--color-accent);
 }
 
-/* --------------------------------------------------------
- * Task-Modal (Erstellen / Bearbeiten)
- * -------------------------------------------------------- */
-.modal-backdrop {
-  position: fixed;
-  inset: 0;
-  background-color: var(--color-overlay);
-  z-index: var(--z-modal);
-  display: flex;
-  align-items: flex-end;
-  justify-content: center;
-  animation: backdrop-in 0.2s ease;
-}
-
-@media (min-width: 640px) {
-  .modal-backdrop {
-    align-items: center;
-  }
-}
-
-@keyframes backdrop-in {
-  from { opacity: 0; }
-  to   { opacity: 1; }
-}
-
-.modal {
-  background-color: var(--color-surface);
-  border-radius: var(--radius-lg) var(--radius-lg) 0 0;
-  width: 100%;
-  max-height: 92dvh;
-  overflow-y: auto;
-  padding: var(--space-6);
-  padding-bottom: calc(var(--space-6) + var(--safe-area-inset-bottom));
-  animation: modal-in 0.25s ease;
-}
-
-@media (min-width: 640px) {
-  .modal {
-    border-radius: var(--radius-lg);
-    max-width: 540px;
-    max-height: 85dvh;
-    padding-bottom: var(--space-6);
-  }
-}
-
-@keyframes modal-in {
-  from { transform: translateY(24px); opacity: 0; }
-  to   { transform: translateY(0);    opacity: 1; }
-}
-
-.modal__header {
-  display: flex;
-  align-items: center;
-  justify-content: space-between;
-  margin-bottom: var(--space-5);
-}
-
-.modal__title {
-  font-size: var(--text-xl);
-  font-weight: var(--font-weight-bold);
-}
-
-.modal__close {
-  width: 32px;
-  height: 32px;
-  border-radius: var(--radius-full);
-  background-color: var(--color-surface-2);
-  display: flex;
-  align-items: center;
-  justify-content: center;
-  cursor: pointer;
-  min-height: unset;
-  color: var(--color-text-secondary);
-}
-
-.modal__actions {
-  display: flex;
-  gap: var(--space-3);
-  margin-top: var(--space-6);
-}
-
-.modal__actions .btn { flex: 1; }
-
 /* --------------------------------------------------------
  * Overdue-Badge (Navigation)
  * -------------------------------------------------------- */