feat: improve setup wizard UX, add cluster sync error handling, and conditional demo data seeding

Add SEED_DEMO_DATA environment variable to control demo data population, enhance setup wizard with welcome screen and theme toggle, add smooth animations for wizard transitions, improve dashboard endpoint to return structured objects for last_syncs and faulty_nodes instead of raw models, implement comprehensive error handling in cluster sync with failed status tracking and audit logging, fix Proxmox provider

backend/app/core/config.py

@@ -18,6 +18,7 @@ class Settings(BaseSettings):
     jwt_refresh_token_days: int = 14
     token_encryption_key: str = "dev-only-change-me"
     demo_admin_password: str = "ChangeMe_UseEnvInstead"
+    seed_demo_data: bool = False
     cors_origins: list[AnyHttpUrl] | list[str] = ["http://localhost:5173", "http://localhost:8080"]
 
     @field_validator("cors_origins", mode="before")