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)

units/runit/fenris-collect/run

@@ -9,7 +9,7 @@
 #
 # Guarantees (must match systemd timer semantics):
 #   - Initial 2-minute boot delay (sleep 120 before first collect)
-#   - Completion-relative 5-minute cadence (sleep 300 after collect)
+#   - Completion-relative 3-minute cadence (sleep 180 after collect)
 #   - Bounded execution (timeout 90s on each collect)
 #   - No catch-up (fixed sleep interval, no Persistent=)
 #   - Serialized runs (runsv does not restart until exit)
@@ -20,7 +20,7 @@ set -eu
 
 COLLECT_TIMEOUT=90
 BOOT_DELAY=120
-CADENCE=300
+CADENCE=180
 LOCK_FILE=/var/lib/fenris/fenris-collect.lock
 
 # Ensure lock directory exists