feat: tip confirmation animation with haptic feedback

Success overlay with animated checkmark and 'Dein Tipp ist drin! 🎯'
message. Haptic vibration on mobile. Auto-closes after 1.2s.

- Add showSuccess state to TipModal
- Trigger vibration feedback on successful submit
- Display success overlay with popIn animation for checkmark
- Auto-close modal after success animation completes
- Add CSS animations (fadeIn, popIn) to TipModal.module.css

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/TipModal.module.css

@@ -300,3 +300,42 @@
 }
 
 .cancelBtn:hover { color: var(--text-secondary); }
+
+/* Success overlay animation */
+.successOverlay {
+  position: absolute;
+  inset: 0;
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+  background: color-mix(in srgb, var(--bg-deep) 92%, transparent);
+  border-radius: inherit;
+  animation: fadeIn 0.3s ease;
+  z-index: 10;
+}
+
+.successCheck {
+  width: 64px;
+  height: 64px;
+  border-radius: 50%;
+  background: var(--success);
+  color: white;
+  font-size: 32px;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  animation: popIn 0.4s cubic-bezier(0.175, 0.885, 0.32, 1.275);
+}
+
+.successText {
+  margin-top: 12px;
+  font-size: 1.1rem;
+  font-weight: 600;
+  color: var(--text-primary);
+}
+
+@keyframes popIn {
+  0% { transform: scale(0); opacity: 0; }
+  100% { transform: scale(1); opacity: 1; }
+}