fix: resolve pnpm frozen-lockfile error and argon2 native build

- Run pnpm install to update lockfile with packages/backend dependencies
- Add argon2 to root onlyBuiltDependencies (pnpm-workspace.yaml + package.json)
- Add explicit `pnpm rebuild argon2` in Dockerfile.backend to ensure native
  bindings compile regardless of pnpm v10 build approval state
- Remove pnpm.onlyBuiltDependencies from packages/backend/package.json
  (ineffective in workspace packages, warned by pnpm)

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

pnpm-workspace.yaml

@@ -5,4 +5,5 @@ ignoredBuiltDependencies:
     - "@tailwindcss/oxide"
 
 onlyBuiltDependencies:
+    - argon2
     - svelte-preprocess