fix: align settings workspace shell

[shuxueshuxue]

Summary

• add router-local helpers for workspace path normalization and recent-workspace list updates
• reuse them from set_default_workspace and add_recent_workspace without flattening their route-specific semantics
• keep repo/local persistence ownership inside each route and leave add_recent_workspace unable to mutate default_workspace

Verification

• python3 -m py_compile backend/web/routers/settings.py
• uv run ruff check backend/web/routers/settings.py