feat(ux): epic UX design improvements across agents, chat, notes, and i18n

Comprehensive UI/UX updates including agent card redesign, chat container
improvements, note editor enhancements, memory echo notifications, and
updated translations for all 15 locales.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

keep-notes/app/(main)/agents/page.tsx

@@ -19,7 +19,7 @@ export default async function AgentsPage() {
   ])
 
   return (
-    <div className="flex-1 flex flex-col h-full bg-slate-50/50">
+    <div className="flex-1 flex flex-col h-full bg-background">
       <div className="flex-1 p-8 overflow-y-auto">
         <div className="max-w-6xl mx-auto">
           <AgentsPageClient