chore: remove custom install phase, nixpacks default is sufficient

supportedArchitectures in package.json ensures the linux binding is in
the lockfile, so a standard frozen install now works correctly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nixpacks.toml

@@ -1,8 +1,5 @@
 [phases.setup]
 nixPkgs = ["nodejs_23", "pnpm"]
 
-[phases.install]
-cmds = ["pnpm install --no-frozen-lockfile --force"]
-
 [phases.build]
 cmds = ["pnpm build"]