Add WebUI env loading and backend preflight

[summerview1997]

Summary

This PR fixes WebUI training launches that silently miss local model endpoint
configuration.

The WebUI previously built the subprocess environment from os.environ plus
.secrets/*.env, but did not load the repository-level .env. A user could
successfully test a Qwen/OpenAI-compatible endpoint from the shell while the
WebUI subprocess fell back to defaults such as http://localhost:8000/v1.

Changes

• Load PROJECT_ROOT/.env before .secrets/*.env when building the WebUI
  training subprocess environment.
• Support common export KEY=value syntax in env files.
• Reuse the same effective environment for preflight validation and subprocess
  launch.
• Add preflight errors for missing openai_chat endpoints.
• Add preflight connectivity checks for configured qwen_chat base URLs.
• Add unit tests for .env loading and backend preflight behavior.

Impact

Users get an immediate actionable error before launch when a backend endpoint is
missing or unreachable, instead of a training run that appears to proceed while
all rollouts fail.

Validation

• /home/thomas/SkillOpt/.venv/bin/python -m pytest -q tests/test_webui_env_preflight.py
• /home/thomas/SkillOpt/.venv/bin/python -m pytest -q
• /home/thomas/SkillOpt/.venv/bin/python -m ruff check skillopt_webui/app.py tests/test_webui_env_preflight.py
• /home/thomas/SkillOpt/.venv/bin/python -m py_compile skillopt_webui/app.py tests/test_webui_env_preflight.py
• git diff --check

[summerview1997]

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