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/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 {