fix(stellar): clamp balance cash at zero

[sozidatel]

Summary

• Clamp treasury cash calculation in /balance so the "Наличных" column never goes below zero
• Add a focused test for overpaid treasury rows and totals
• Add font fallback for local balance image generation on macOS/Linux

Tests

• POSTGRES_URL=postgresql+asyncpg://user:password@localhost:5432/skynet_db REDIS_URL=redis://localhost:6379/4 .venv/bin/pytest tests/other/stellar/test_display_commands.py tests/other/test_img_tools.py tests/routers/test_stellar.py::test_show_balance_command -q
• .venv/bin/ruff check other/stellar/display_commands.py other/img_tools.py tests/other/stellar/test_display_commands.py tests/other/test_img_tools.py