refactor: migrate session, script-builder, account to Design System v4

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

frontend/src/components/script-builder/ScriptBuilderInput.tsx

@@ -55,7 +55,7 @@ export function ScriptBuilderInput({
         rows={1}
         className={cn(
           "flex-1 resize-none rounded-xl px-4 py-2.5 text-sm",
-          "bg-card border border-border text-foreground placeholder:text-muted-foreground",
+          "bg-[#14161d] border border-[#1e2130] text-[#e2e5eb] placeholder:text-[#848b9b]",
           "focus:outline-none focus:border-[rgba(6,182,212,0.3)] transition-colors",
           "disabled:opacity-50"
         )}
@@ -67,7 +67,7 @@ export function ScriptBuilderInput({
         className={cn(
           "shrink-0 flex items-center justify-center w-10 h-10 rounded-xl transition-all",
           canSend
-            ? "bg-gradient-brand text-[#101114] hover:opacity-90 active:scale-[0.97]"
+            ? "bg-[#22d3ee] text-white hover:brightness-110 active:scale-[0.98]"
             : "bg-[rgba(255,255,255,0.04)] text-[#5a6170] cursor-not-allowed"
         )}
       >