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

package.json

@@ -28,6 +28,19 @@
   "devDependencies": {
     "concurrently": "^8.2.2"
   },
+  "overrides": {
+    "elliptic": "^6.6.1",
+    "lodash": "^4.17.21",
+    "esbuild": "^0.25.0",
+    "bigint-buffer": "^1.1.5",
+    "nanoid": "^3.3.8",
+    "minimatch": "^5.1.6",
+    "js-yaml": "^4.1.0",
+    "serialize-javascript": "^7.0.5",
+    "@walletconnect/utils": "^2.20.0",
+    "@walletconnect/core": "^2.20.0",
+    "@walletconnect/sign-client": "^2.20.0"
+  },
   "workspaces": [
     "backend",
     "frontend",