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_status_composition.py

@@ -870,7 +870,7 @@ class TestRefreshTiming:
         _insert_segment(conn)
         _open_period(conn)
         now = _clock()
-        ts = (now - timedelta(minutes=10)).isoformat()
+        ts = (now - timedelta(minutes=4)).isoformat()
         _insert_sample(conn, ts)
         for i in range(20):
             d = (datetime(2026, 9, 10) + timedelta(days=i)).strftime("%Y-%m-%d")
@@ -878,27 +878,27 @@ class TestRefreshTiming:
 
         svc = {
             "boot_enabled": True, "timer_active": True,
-            "last_collect_ok": True, "last_collect_age_s": 600,
+            "last_collect_ok": True, "last_collect_age_s": 240,
             "last_collect_reason": None,
         }
 
-        # Sample is 10 min old (600s) → fresh (within 690s threshold)
+        # Sample is 4 min old (240s) → fresh (within 450s threshold)
         comp1 = compose_status(conn, svc, now, store_fault=None, newer_schema=None)
         assert comp1.freshness == "fresh"
         assert comp1.state == StatusState.MONITORING
 
-        # At now + 1 min → sample is 11 min old (660s) → still fresh
-        now2 = now + timedelta(minutes=1)
+        # At now + 3 min → sample is 7 min old (420s) → still fresh
+        now2 = now + timedelta(minutes=3)
         comp2 = compose_status(conn, svc, now2, store_fault=None, newer_schema=None)
         assert comp2.freshness == "fresh"
 
-        # At now + 2 min → sample is 12 min old (720s) → missed (past 690s)
-        now3 = now + timedelta(minutes=2)
+        # At now + 5 min → sample is 9 min old (540s) → missed (past 450s)
+        now3 = now + timedelta(minutes=5)
         comp3 = compose_status(conn, svc, now3, store_fault=None, newer_schema=None)
         assert comp3.freshness == "missed"
         assert comp3.state == StatusState.MONITORING
 
-        # At now + 49 hours → sample is 49h 10min old → stale
+        # At now + 49 hours → sample is 49h 4min old → stale
         now4 = now + timedelta(hours=49)
         comp4 = compose_status(conn, svc, now4, store_fault=None, newer_schema=None)
         assert comp4.freshness == "stale"