Clarify project session hierarchy in the sidebar

[xiami762]

Summary

• visually nest project sessions with the same outlined card treatment used by task sessions
• remove per-project disclosure icons and keep project-row expand/collapse behavior accessible
• add local project sharing that exposes the project and all existing or future sessions as read-only to other local users
• allow project owners to share or unshare from the project menu while hiding write actions for viewers
• start new projects with an empty folder field and open the folder browser from the user home directory
• save directly with the current browsed folder when the explicit folder-selection button was not clicked
• prevent Enter in the project name field from submitting before a folder is available, including IME composition
• avoid duplicate project folder paths while the folder browser is open

Validation

• .venv/bin/python -m pytest tests/project/test_project.py tests/server/routes/test_project_routes.py tests/server/routes/test_session_routes.py -q (81 passed)
• npm test -- --run src/pages/Session/index.test.tsx (47 passed)
• npx eslint src/pages/Session/index.tsx src/pages/Session/index.test.tsx --report-unused-disable-directives --max-warnings 0
• npm run build
• .venv/bin/ruff check flocks/project/project.py flocks/server/routes/project.py flocks/server/routes/session.py flocks/session/policy.py tests/server/routes/test_project_routes.py
• git diff --check