feat: add admin user management page with CRUD and i18n

- New admin blueprint with CRUD proxy endpoints for users
- Admin user management template with search, create, edit, toggle active
- Navbar: add admin link for is_admin users (desktop + mobile)
- Register admin blueprint in app factory
- Add IT/EN translations for all admin UI strings

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

client/app.py

@@ -37,11 +37,13 @@ def create_app() -> Flask:
     from blueprints.measure import measure_bp
     from blueprints.maker import maker_bp
     from blueprints.statistics import statistics_bp
+    from blueprints.admin import admin_bp
 
     app.register_blueprint(auth_bp)
     app.register_blueprint(measure_bp, url_prefix="/measure")
     app.register_blueprint(maker_bp, url_prefix="/maker")
     app.register_blueprint(statistics_bp, url_prefix="/statistics")
+    app.register_blueprint(admin_bp, url_prefix="/admin")
 
     @app.route("/")
     def index():