Improve MCP token user linking

[pilipilisbot]

Summary

• replace the admin MCP owner free-text field with a selector populated from known dashboard users
• add admin-only MCP token owner reassignment so existing active tokens can be linked to a user
• reject typo/unknown owners at the API layer and document the operational behavior

Fixes #180

Screenshots

Verification

• . ../github-agent-bridge/.venv/bin/activate && pytest -q tests/test_mcp.py tests/test_backend.py (72 passed)
• . ../github-agent-bridge/.venv/bin/activate && pytest -q (350 passed)
• npm test -- --run src/main.test.tsx (50 passed)
• npm run build