fix(build): add next-themes + sql.js deps and mark sql.js as webpack external

- next-themes: required by TierFlowDiagram.tsx (onboarding)
- sql.js: required by CLI runtime sqliteRuntime.mjs
- Add sql.js to serverExternalPackages in next.config.mjs to prevent
  webpack static resolution failures during build

next.config.mjs

@@ -111,6 +111,7 @@ const nextConfig = {
     "thread-stream",
     "pino-abstract-transport",
     "better-sqlite3",
+    "sql.js",
     "keytar",
     "wreq-js",
     "zod",