feat(web): update logo to new A-mark design, match accent color palette

- New AresLogo component: A-shape with indigo gradient (#5e6ad2 → #828fff)
- Replaces old square icon in Nav, footer, and favicon.svg
- Colors match site accent tokens (c-accent / c-accent-hov)

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

web/components/Nav.tsx

@@ -3,6 +3,7 @@ import Link from "next/link";
 import { usePathname } from "next/navigation";
 import clsx from "clsx";
 import { GITHUB_URL } from "@/lib/github";
+import AresLogo from "./AresLogo";
 
 const links = [
   { href: "/",         label: "Dashboard" },
@@ -17,11 +18,7 @@ export default function Nav({ version }: { version?: string | null }) {
       <div className="max-w-5xl mx-auto px-4 sm:px-6 h-14 flex items-center gap-6">
         {/* logo + wordmark */}
         <Link href="/" className="flex items-center gap-2 group">
-          <svg width="26" height="26" viewBox="0 0 26 26" fill="none" aria-hidden="true">
-            <rect width="26" height="26" rx="6" fill="#5e6ad2"/>
-            <path d="M8 19 L13 8 L18 19" stroke="white" strokeWidth="1.8" strokeLinecap="round" strokeLinejoin="round"/>
-            <path d="M10.2 15.5 H15.8" stroke="white" strokeWidth="1.8" strokeLinecap="round"/>
-          </svg>
+          <AresLogo size={28} />
           <span className="font-mono text-sm font-semibold tracking-widest text-ink group-hover:text-accent transition-colors" style={{ letterSpacing: "0.18em" }}>
             ARES
           </span>