Refresh the look and layout of the model-usage plugin

shell/plugins/model-usage/scripts/codex_usage_scanner.py

@@ -71,6 +71,7 @@ today_tokens_by_model = {}
 model_usage = {}
 sessions_by_day = {day: set() for day in recent_dates}
 today_sessions = set()
+active_days = set()
 
 today_prompts = 0
 today_total_tokens = 0
@@ -86,6 +87,7 @@ def add_usage(day, session_key, model, input_tokens, output_tokens, cache_read,
   total = input_tokens + output_tokens + cache_read + cache_write
   total_prompts += 1
   total_sessions.add(session_key)
+  active_days.add(day)
 
   bucket = model_usage.setdefault(model, {
     "inputTokens": 0,
@@ -336,6 +338,11 @@ out = {
   "recentDays": [recent[day] for day in recent_dates],
   "totalPrompts": total_prompts,
   "totalSessions": len(total_sessions),
+  # Days with any recorded usage, for the all-time "N days" summary.
+  # The dates travel too: merging snapshots from several machines needs
+  # their union, which a count alone cannot give.
+  "activeDays": len(active_days),
+  "activeDates": sorted(active_days),
   "modelUsage": model_usage,
   "usageStatusText": usage_status,
   "authHelpText": usage_help,