Upgrade Next.js to 15.5.27 (critical RCE advisories) (#264)

next and eslint-config-next pinned to 15.5.27 (was ^15.0.0 locked at
15.5.19). Fixes GHSA-2xp9-vwfh-vxw4 (Image Optimization RCE).

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: ralyodio

bun.lock

@@ -38,7 +38,7 @@
     "@vitest/ui": "^4.1.8",
     "chai": "^6.0.1",
     "eslint": "^9.39.2",
-    "eslint-config-next": "^15.0.0",
+    "eslint-config-next": "15.5.27",
     "fake-indexeddb": "^6.2.5",
     "globals": "^17.7.0",
     "jsdom": "^26.1.0",
@@ -66,7 +66,7 @@
     "form-data": "^4.0.4",
     "mailgun.js": "^12.0.3",
     "mlkem": "^2.5.0",
-    "next": "^15.0.0",
+    "next": "15.5.27",
     "openpgp": "^6.2.0",
     "posthog-js": "^1.381.0",
     "react": "^19.0.0",