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) */