feat: allow for manual color entry

[hazedav]

Summary

• Add option to enter a hex color code manually instead of selecting a league/team, allowing users to find filament matches for any arbitrary color
• Add get_manual_hex_color() for hex input with validation and normalization (accepts with/without #, case-insensitive)
• Add display_manual_color() to show exact and similar filament matches for manually entered colors, with the same weighted scoring, purchase link, and top manufacturer logic as team color lookups
• Add test suite for main.py CLI functions (38 tests covering league selection, manual hex entry, filament type selection, and display functions)

Test plan

• make test — all 438 tests pass
• make lint — no linting errors
• Manual testing: run CLI, select option 0, enter a hex code, verify filament matches display correctly

🤖 Generated with Claude Code