fix(brand): replace Tomcat default favicon.ico with KINTEX-branded icon

Root cause of persistent Tomcat cat favicon: public/favicon.ico was the
Tomcat default (21630 bytes), shipped by every build; browsers prefer
/favicon.ico over the correct favicon.png. Regenerate favicon.ico from the
KINTEX wordmark PNG (multi-size 16/32/48/64) and bump cache-bust to v3.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/frontend/index.html

@@ -4,9 +4,9 @@
     <meta charset="UTF-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <title>KINTEX AI 전시·행사시스템</title>
-    <link rel="icon" href="/favicon.ico?v=2" sizes="any" />
-    <link rel="icon" type="image/png" sizes="128x128" href="/favicon.png?v=2" />
-    <link rel="apple-touch-icon" href="/favicon.png?v=2" />
+    <link rel="icon" href="/favicon.ico?v=3" sizes="any" />
+    <link rel="icon" type="image/png" sizes="128x128" href="/favicon.png?v=3" />
+    <link rel="apple-touch-icon" href="/favicon.png?v=3" />
     <meta name="theme-color" content="#0066b3" />
     <link rel="preconnect" href="https://cdn.jsdelivr.net" crossorigin />
     <link