Fix history loading, add Dual-View mode and Model Management

[google-labs-jules[bot]]

1. History Loading Fix: Updated backend chat_api.py to include steps in execution_details when saving history. Updated frontend App.jsx to correctly load steps from execution.steps or execution_details.steps.
2. Dual-View Mode: Implemented a User/Developer mode toggle in the frontend. "User Mode" simplifies the output (hiding SQL/JSON), while "Developer Mode" shows full details.
3. Stop Button: Replaced the "Send" button with a "Stop" button during generation, wired to /api/stop_query.
4. Model Management: Added a UI in SettingsModal.jsx to add and delete models directly from the frontend.
5. Internationalization (i18n): Added LanguageContext, English/Chinese locale files, and replaced hardcoded strings in the UI.
6. Bug Fixes: Fixed a missing Brain import in App.jsx that caused a crash. Fixed LanguageContext default logic.

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PR created automatically by Jules for task 2194923725017469909 started by @MKY508

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