security: update Next.js to 15.5.11 to fix vulnerabilities

Fixes 3 Dependabot security alerts:
- CVE for HTTP request deserialization DoS (high severity)
- CVE for unbounded memory consumption via PPR Resume Endpoint (medium)
- CVE for DoS via Image Optimizer remotePatterns (medium)

Author: brandonkachen

bun.lock

@@ -72,7 +72,7 @@
     "framer-motion": "^11.13.3",
     "lucide-react": "^0.487.0",
     "mermaid": "^11.8.1",
-    "next": "15.5.9",
+    "next": "15.5.11",
     "next-auth": "^4.24.11",
     "next-contentlayer2": "^0.5.8",
     "next-themes": "^0.3.0",