Introduce --color-danger-solid-* CSS variables and replace hardcoded values

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

frontend/src/assets/main.css

@@ -25,6 +25,9 @@
   --color-danger-bg-strong: rgba(220, 38, 38, 0.2);
   --color-danger-border: rgba(220, 38, 38, 0.3);
   --color-danger-border-strong: rgba(220, 38, 38, 0.4);
+  --color-danger-solid: #d32f2f;
+  --color-danger-solid-hover: #b71c1c;
+  --color-danger-solid-text: #fff;
 
   /* Glass system */
   --color-glass: rgba(255, 255, 255, 0.1);
@@ -214,7 +217,7 @@ textarea.form-field {
 
 /* Error message */
 .field-error {
-  color: #fff;
+  color: var(--color-danger-solid);
   font-size: 0.875rem;
   font-weight: 600;
   padding-left: 0.25rem;
@@ -334,7 +337,7 @@ textarea.form-field {
 }
 
 .rsvp-form__field-error {
-  color: #d32f2f;
+  color: var(--color-danger-solid);
   font-size: 0.875rem;
   font-weight: 600;
   padding-left: 0.25rem;

frontend/src/components/ConfirmDialog.vue

@@ -139,8 +139,8 @@ watch(
 }
 
 .confirm-dialog__btn--confirm {
-  background: #d32f2f;
-  color: #fff;
+  background: var(--color-danger-solid);
+  color: var(--color-danger-solid-text);
 }
 
 .confirm-dialog-enter-active,

frontend/src/components/EventCard.vue

@@ -175,7 +175,7 @@ function onTouchEnd() {
 }
 
 .event-card__delete:hover {
-  color: #d32f2f;
+  color: var(--color-danger-solid);
   background: rgba(211, 47, 47, 0.08);
 }
 

frontend/src/views/EventDetailView.vue

@@ -677,15 +677,15 @@ onMounted(fetchEvent)
   font-family: inherit;
   font-size: 1rem;
   font-weight: 700;
-  color: #fff;
-  background: #d32f2f;
+  color: var(--color-danger-solid-text);
+  background: var(--color-danger-solid);
   border: none;
   cursor: pointer;
   transition: background 0.15s ease;
 }
 
 .cancel-form__confirm:hover {
-  background: #b71c1c;
+  background: var(--color-danger-solid-hover);
 }
 
 .cancel-form__confirm:disabled {