Implement Amber/Nord/High Contrast theme presets with XDG user-scoped
persistence and reduced motion toggle. Covers TPH-10 and preference
integration with TPH-2.
- preferences.py: safe load/save with XDG_CONFIG_HOME/fenris/preferences.json
- themes.py: three Textual Theme objects with graph colour roles
- TUI: t cycles presets, m toggles reduced motion, both persist across restart
- Status composition receives reduced_motion from preferences
- 56 new tests covering persistence, themes, TUI integration, CLI isolation
- All 590 existing tests continue to pass
