fix(ui): take the agentic turns default from DEFAULT_AGENTIC_CONFIG
The registry hard-coded 10 for AGENTIC_MAX_TURNS while DEFAULT_AGENTIC_CONFIG
said 100. The registry seeds SETTING_CONFIG_DEFAULT, so the loop's
`|| DEFAULT_AGENTIC_CONFIG.maxTurns` fallback never fires and users got 10
turns. Reference the shared constant, as the adjacent MCP timeout does.
Assisted by: llama-ui (ggml-org/Qwen3.8-Flash-Next:IQ4_NL)