test: add manual WebUI branch upgrade flow

[xiami762]

Summary

• Add a manual, opt-in WebUI upgrade test that can target a confirmed branch archive without changing production update routes.
• Build the WebUI before starting the frontend dev server in scripts/dev.sh.

Validation

• uv run pytest tests/server/routes/test_update_routes.py
• uv run ruff check tests/server/routes/test_update_routes.py flocks/server/routes/update.py
• bash -n scripts/dev.sh
• git diff --check