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)

src/fenris/init_system.py

@@ -10,13 +10,13 @@ responsibility shared by the privileged control path and read-only status
 composition.
 
 Runit service layout:
-  /etc/sv/fenris-collect/run        — scheduler (sleep 120; loop { collect; sleep 300 })
+  /etc/sv/fenris-collect/run        — scheduler (sleep 120; loop { collect; sleep 180 })
   /etc/sv/fenris-collect/log/run    — logger to /var/log/fenris-collect/
   /var/service/fenris-collect       — symlink to enable
   /etc/sv/fenris-collect/down       — marker for dormant install
 
 Runit guarantees:
-  - Completion-relative 5-minute cadence (sleep 300 after each collect)
+  - Completion-relative 3-minute cadence (sleep 180 after each collect)
   - Initial 2-minute boot delay (sleep 120 before first collect)
   - Bounded execution (90s timeout via timeout(1))
   - No catch-up (service sleeps fixed interval, no Persistent= flag)