fix: resolve langchain dependency conflicts and improve setup instructions

[koldunovn]

Summary

• Removed langchain-classic from pip dependencies in environment.yml — it requires langchain-core>=1.2.19 which overwrites the conda-installed langchain-core<1.0.0 and breaks langchain 0.3.x, langchain-openai, and langchain-experimental. The package is only used as a fallback import (except ImportError), so the conda-provided langchain already covers it.
• Pinned langchain-anthropic<1.0.0 to keep it compatible with the 0.3.x LangChain ecosystem.
• Reordered README setup steps so npm install runs before uvicorn --reload, preventing uvicorn's file watcher from detecting node_modules/ creation and triggering a restart that causes ETIMEDOUT proxy errors in the frontend.
• Added --reload-exclude 'frontend/node_modules/*' to the uvicorn command as a safety net.

Test plan

• Run micromamba env create -f environment.yml from scratch — no dependency conflict errors
• Follow README steps in order — no ETIMEDOUT errors on frontend startup
• Verify import langchain and import langchain_anthropic work in the created environment

[dmpantiu]

LGTM. Note: pyproject.toml and requirements.txt still list langchain-classic — consider updating those for consistency in a follow-up.