feat: global time range context with custom date picker

Lift time range state into a shared React context so the selected
range persists across page navigation and browser refreshes
(localStorage). Add a "Custom" option with a popover date picker
that lets users specify arbitrary start/end dates. All preset end
times now use endOf("day") (23:59:59) instead of the current moment.

lib/i18n.tsx

@@ -26,6 +26,10 @@ const translations = {
     "time.7d": "7 天",
     "time.30d": "30 天",
     "time.all": "全部",
+    "time.custom": "自定义",
+    "time.startDate": "开始日期",
+    "time.endDate": "结束日期",
+    "time.confirm": "确认",
     // granularity
     "gran.day": "日",
     "gran.week": "周",
@@ -108,6 +112,10 @@ const translations = {
     "time.7d": "7 Days",
     "time.30d": "30 Days",
     "time.all": "All",
+    "time.custom": "Custom",
+    "time.startDate": "Start",
+    "time.endDate": "End",
+    "time.confirm": "Confirm",
     "gran.day": "Day",
     "gran.week": "Week",
     "gran.month": "Month",