feat(i18n): add English/Chinese language toggle with full UI translation

[koyouko]

Summary

This PR combines two major improvements:

1. Multi-provider LLM support — backend now supports 5 providers (Anthropic/Claude, OpenAI, GitHub Copilot, Ollama, MiniMax/GLM)
2. Full English/Chinese i18n — frontend defaults to English with a live language toggle

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1. Multi-Provider LLM Backend

New Files

File	Purpose
backend/app/utils/copilot_auth.py	GitHub Copilot OAuth token exchange (from upstream PR #383)

Modified Files

File	Changes
backend/app/utils/llm_client.py	Unified 5-provider client with estimate_tokens() and chat_with_tools()
backend/app/config.py	All provider configs + English comments
backend/app/services/simulation_runner.py	Thread-safety fixes (from upstream PR #389)
backend/app/services/oasis_profile_generator.py	Updated to use unified client
backend/app/services/simulation_config_generator.py	Updated to use unified client
backend/app/__init__.py	Translated to English
backend/app/api/__init__.py	Translated to English
.env.example	All provider env vars documented
backend/requirements.txt / pyproject.toml	Updated dependencies

Supported Providers

LLM_PROVIDER=anthropic   # Claude (claude-opus-4-5, claude-sonnet-4-5, etc.)
LLM_PROVIDER=openai      # OpenAI (gpt-4o, gpt-4-turbo, etc.)
LLM_PROVIDER=copilot     # GitHub Copilot (OAuth token)
LLM_PROVIDER=ollama      # Local Ollama (llama3, mistral, etc.)
LLM_PROVIDER=minimax     # MiniMax/GLM (original default)

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2. Frontend i18n (English/Chinese Toggle)

New Files

File	Purpose
frontend/src/store/locale.js	Reactive locale singleton backed by localStorage
frontend/src/i18n/index.js	Full en + zh translation dictionaries
frontend/src/i18n/useT.js	useT() composable — computed key-resolver, auto re-renders on locale change

All 15 Vue files translated

Views: Home.vue, MainView.vue, SimulationView.vue, SimulationRunView.vue, ReportView.vue, InteractionView.vue, Process.vue

Step components: Step1GraphBuild.vue, Step2EnvSetup.vue, Step3Simulation.vue, Step4Report.vue, Step5Interaction.vue

Other: GraphPanel.vue, HistoryDatabase.vue

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Test Plan

• English shown by default on page load
• EN/中 toggle switches entire UI instantly, persists to localStorage
• Production build passes (vite build — 679 modules, no errors)
• LLM_PROVIDER=anthropic routes to Claude API correctly
• Thread-safe simulation runner (no race conditions on concurrent requests)

[666ghj]

The project already has i18n support in place — please refer to [PR #428] for reference.