security: upgrade dependencies to fix critical and high vulnerabilities

- next: 14.2.15 → 15.2.5 (fixes critical DoS, SSRF, cache poisoning)
- react/react-dom: 18.2.0 → 19.1.0
- @solana/spl-token: 0.3.9 → 0.4.14 (bigint-buffer overflow fix)
- vitest: 1.1.0 → 3.0.0 (esbuild dev server exposure fix)
- mocha: 9.0.3 → 11.7.5 (serialize-javascript RCE fix)
- @solana/wallet-adapter-*: latest versions
- Add npm overrides for elliptic, lodash, esbuild, nanoid

Vulnerabilities: 44 (1 critical) → 23 (3 high, 20 low)

Refs: npm audit fixes for GHSA-7m27-7ghc-44w9, GHSA-3gc7-fjrx-p6mg

backend/package.json

@@ -16,7 +16,7 @@
   },
   "dependencies": {
     "@prisma/client": "^5.7.0",
-    "@solana/spl-token": "^0.3.9",
+    "@solana/spl-token": "^0.4.14",
     "@solana/web3.js": "^1.87.6",
     "axios": "^1.6.2",
     "bcryptjs": "^2.4.3",
@@ -44,6 +44,6 @@
     "prisma": "^5.7.0",
     "tsx": "^4.7.0",
     "typescript": "^5.3.3",
-    "vitest": "^1.1.0"
+    "vitest": "^3.0.0"
   }
 }