Plot live drive activity every three minutes (#91)

- Change default collection cadence from 5 minutes to 3 minutes
  (CADENCE_DEFAULT_S=180, systemd OnUnitInactiveSec=3min,
  runit CADENCE=180)
- Update freshness threshold to 450s (2×180 + AccuracySec + 60)
- Add _query_live_graph_data(): queries raw samples from the last
  3 hours and computes interval byte deltas with actual timestamps
- Add LiveActivityGraph widget: vertical bar chart of interval
  volumes with read/write toggle (w key), arrow key inspection,
  and click support
- Wire live graph into TUI layout (full-width row between daily
  graph and drive health), refresh cycle, and CSS grid
- Replace t theme binding with t today/live binding; theme
  selection via preferences file
- Add w binding for read/write toggle on live graph
- Update action legend, help screen, and grid layout for new
  live-activity row
- Add 20 tests covering cadence constants, live query, widget
  rendering, toggle, and TUI integration
- Update all cadence documentation (README, ADR 0003, acceptance
  criteria LC-2, fenris-redesign constants table, CHANGELOG)

tests/test_tui.py

@@ -376,7 +376,7 @@ class TestDenseScreen:
                 main_grid = app.query_one("#main-grid")
                 assert str(main_grid.styles.layout) == "<grid>"
                 assert main_grid.has_class("paused")
-                assert len(main_grid.styles.grid_rows) == 5
+                assert len(main_grid.styles.grid_rows) == 6
                 banner = str(paused_banner.render()).lower()
                 assert "monitoring: paused — deliberate disable" in banner
                 assert "paused time is excluded from your usage habit · resume: fenris monitor resume" in banner