feat(ux): microinteraction polish — undo tap feedback, strikethrough transition, modal loading state

- toast__undo: add :active scale + tap-highlight-color for reliable tap feedback
- task titles: animate text-decoration-color instead of snapping for smoother done-state
- modal forms: auto-add btn--loading on submit; rAF guard removes it on validation fail;
  MutationObserver removes it on error re-enable; btnSuccess clears it before checkmark

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

public/components/modal.js

@@ -294,6 +294,22 @@ export function openModal({ title, content, onSave, onDelete, size = 'md' } = {}
   // Callback für Aufrufer (Form-Events binden etc.)
   if (typeof onSave === 'function') onSave(panel);
 
+  // Loading-State: btn--loading auf Submit-Button während async-Save.
+  // rAF-Check: Validierung schlägt fehl → btn bleibt enabled → Loading sofort entfernen.
+  // MutationObserver: Error-Pfad → btn wird re-enabled → Loading entfernen.
+  panel.addEventListener('submit', (e) => {
+    const btn = e.target.querySelector('[type="submit"], .btn--primary');
+    if (!btn || btn.disabled) return;
+    btn.classList.add('btn--loading');
+    requestAnimationFrame(() => {
+      if (!btn.disabled) { btn.classList.remove('btn--loading'); return; }
+      const mo = new MutationObserver(() => {
+        if (!btn.disabled) { btn.classList.remove('btn--loading'); mo.disconnect(); }
+      });
+      mo.observe(btn, { attributes: true, attributeFilter: ['disabled'] });
+    });
+  }, { capture: true });
+
   // Standalone: Statusbar abdunkeln (Overlay-Effekt)
   if (window.oikos?.setThemeColor) {
     window.oikos.setThemeColor(OVERLAY_THEME_COLOR, OVERLAY_THEME_COLOR);
@@ -620,6 +636,7 @@ export function validateAll(formContainer) {
  * @param {string} [originalLabel]
  */
 export function btnSuccess(btn, originalLabel) {
+  btn.classList.remove('btn--loading');
   const label = originalLabel ?? btn.textContent;
   btn.classList.add('btn--success');
   const reducedMotion = matchMedia('(prefers-reduced-motion: reduce)').matches;

public/styles/layout.css

@@ -1647,10 +1647,20 @@
   opacity: 0.85;
 }
 
+.toast__undo {
+  -webkit-tap-highlight-color: transparent;
+  transition: opacity var(--transition-fast), transform 0.08s ease;
+}
+
 .toast__undo:hover {
   opacity: 1;
 }
 
+.toast__undo:active {
+  transform: scale(0.94);
+  opacity: 1;
+}
+
 .toast--success { background-color: var(--color-success); color: var(--toast-success-text); }
 .toast--danger  { background-color: var(--color-danger);  color: var(--toast-danger-text); }
 .toast--warning { background-color: var(--color-warning); color: var(--toast-warning-text); }

public/styles/tasks.css

@@ -406,11 +406,16 @@
   color: var(--color-text-primary);
   margin-bottom: var(--space-1);
   cursor: pointer;
+  text-decoration: line-through;
+  text-decoration-color: transparent;
+  transition:
+    color var(--transition-fast),
+    text-decoration-color var(--transition-base);
 }
 
 .task-card--done .task-card__title {
-  text-decoration: line-through;
   color: var(--color-text-secondary);
+  text-decoration-color: var(--color-text-secondary);
 }
 
 .task-card__meta {