Remove manual -webkit-backdrop-filter prefixes

LightningCSS (Vite 8) was stripping the unprefixed backdrop-filter when
it saw the manual -webkit- prefix, breaking blur effects in Firefox and
on production. Let LightningCSS handle prefixing automatically.

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

frontend/src/assets/main.css

@@ -244,7 +244,6 @@ textarea.form-field {
   border: 1px solid var(--color-glass-border);
   box-shadow: var(--shadow-card);
   backdrop-filter: blur(16px);
-  -webkit-backdrop-filter: blur(16px);
 }
 
 .glass:hover:not(input):not(textarea):not(.btn-primary) {
@@ -256,7 +255,6 @@ textarea.form-field {
 .glass-inner {
   background: var(--color-glass-inner);
   backdrop-filter: blur(16px);
-  -webkit-backdrop-filter: blur(16px);
 }
 
 /* Glow border: conic gradient wrapper with halo (static) */

frontend/src/components/BottomSheet.vue

@@ -107,7 +107,6 @@ function onTouchEnd() {
   border: 1px solid var(--color-glass-border);
   border-bottom: none;
   backdrop-filter: blur(24px);
-  -webkit-backdrop-filter: blur(24px);
   border-radius: 20px 20px 0 0;
   padding: var(--spacing-lg) var(--spacing-xl) var(--spacing-2xl);
   width: 100%;

frontend/src/components/ConfirmDialog.vue

@@ -87,7 +87,6 @@ watch(
   background: linear-gradient(135deg, var(--color-glass-strong) 0%, var(--color-glass-subtle) 100%);
   border: 1px solid var(--color-glass-border);
   backdrop-filter: blur(24px);
-  -webkit-backdrop-filter: blur(24px);
   border-radius: var(--radius-card);
   box-shadow: 0 8px 32px rgba(0, 0, 0, 0.3);
   padding: var(--spacing-xl);

frontend/src/components/RsvpBar.vue

@@ -154,7 +154,6 @@ watch(expanded, (isExpanded) => {
   background: linear-gradient(135deg, var(--color-glass-strong) 0%, var(--color-glass-subtle) 100%);
   border: 1px solid var(--color-glass-border);
   backdrop-filter: blur(16px);
-  -webkit-backdrop-filter: blur(16px);
   border-radius: var(--radius-card);
   padding: var(--spacing-md) var(--spacing-lg);
   box-shadow: var(--shadow-card);
@@ -206,7 +205,6 @@ watch(expanded, (isExpanded) => {
   background: linear-gradient(135deg, var(--color-glass-strong) 0%, var(--color-glass-subtle) 100%);
   border: 1px solid var(--color-glass-border);
   backdrop-filter: blur(16px);
-  -webkit-backdrop-filter: blur(16px);
   cursor: pointer;
   text-align: center;
   transition: background 0.15s ease;

frontend/src/views/EventDetailView.vue

@@ -496,7 +496,6 @@ onMounted(fetchEvent)
   background: linear-gradient(135deg, var(--color-glass-strong) 0%, var(--color-glass-subtle) 100%);
   border: 1px solid var(--color-glass-border);
   backdrop-filter: blur(16px);
-  -webkit-backdrop-filter: blur(16px);
 }
 
 .detail__meta-text {