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>

package.json

@@ -20,6 +20,7 @@
 	"packageManager": "pnpm@10.19.0",
 	"pnpm": {
 		"onlyBuiltDependencies": [
+			"argon2",
 			"es5-ext",
 			"esbuild",
 			"svelte-preprocess",