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)

CHANGELOG.md

@@ -13,7 +13,7 @@ backfill releases from before this changelog.
 
 ### Changed
 
-- Make usage-history axes, units, UTC boundaries, active 7/14/30/90-day window, gaps, partial periods, stacked write attribution, and hourly drill-down explicit; keep live graph data refreshed on the existing five-minute cadence.
+- Make usage-history axes, units, UTC boundaries, active 7/14/30/90-day window, gaps, partial periods, stacked write attribution, and hourly drill-down explicit; keep live graph data refreshed on the current three-minute cadence.
 
 ## [0.3.6] - 2026-09-16