Persist accessible colour and motion preferences (issue #80)

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

tests/test_tui.py

@@ -423,7 +423,7 @@ class TestDenseScreen:
                 quit_rail = app.query_one("#quit-rail")
                 assert "continuity" in strip
                 assert "monitoring: active in background · persists across reboots" in strip
-                assert "p pause · r resume · c collect · d disclosures" in strip
+                assert "p pause · r resume · c collect · t theme · m motion · d disclosures" in strip
                 assert "q quit" not in strip
                 assert rail == "q QUIT TUI"
                 assert usage.region.y < service.region.y < quit_rail.region.y