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/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%;