Stats: activity heatmap as a rolling window of the last 14 days

One row per calendar day (today at the bottom) instead of weekdays
aggregated over the selected range.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/i18n/en.ts

@@ -714,7 +714,7 @@ export const en = {
   "stats.costPerDay": "Cost per day by model",
   "stats.tokensPerDay": "Tokens per day",
   "stats.heatEmpty": "no activity",
-  "stats.heatmap": "Activity by weekday and hour",
+  "stats.heatmap": "Activity of the last 14 days by hour",
   "stats.byModel": "By model",
   "stats.byProvider": "By provider",
   "stats.byAgent": "By mode",