feat: stats activity-over-time granularity selector (timeline / day / week / month / year)

Backend adds cyclical distributions ByHour (hour-of-day), ByDOW (weekday), ByDOM
(day-of-month) and ByMonthYr (calendar month), all UTC. The Activity-over-time
card gains a selector: Timeline keeps the chronological month trend; Day/Week/
Month/Year show the hour/weekday/day/month distribution as bars.

frontend/src/lib/i18n.tsx

@@ -65,7 +65,7 @@ const en: Dict = {
   'stats.byOperatorSub': '“—” = logged by the station owner (no OPERATOR set)',
   'stats.byStation': 'By station callsign', 'stats.byContinent': 'By continent',
   'stats.topEntities': 'Top DXCC entities', 'stats.byYear': 'By year',
-  'stats.overTime': 'Activity over time', 'stats.overTimeSub': 'QSOs per month',
+  'stats.overTime': 'Activity over time', 'stats.overTimeSub': 'Timeline, or by hour / weekday / day / month', 'stats.granTimeline': 'Timeline', 'stats.granDay': 'Day', 'stats.granWeek': 'Week', 'stats.granMonth': 'Month', 'stats.granYear': 'Year',
   'stats.confirmations': 'Confirmations', 'stats.paperQSL': 'Paper QSL',
   
   'stats.byContinentSub': 'Share of the log',
@@ -379,7 +379,7 @@ const fr: Dict = {
   'stats.byOperatorSub': '« — » = loggé par le titulaire (pas d’OPERATOR renseigné)',
   'stats.byStation': 'Par indicatif de station', 'stats.byContinent': 'Par continent',
   'stats.topEntities': 'Top entités DXCC', 'stats.byYear': 'Par année',
-  'stats.overTime': 'Activité dans le temps', 'stats.overTimeSub': 'QSO par mois',
+  'stats.overTime': 'Activité dans le temps', 'stats.overTimeSub': 'Chronologie, ou par heure / jour sem. / jour / mois', 'stats.granTimeline': 'Chronologie', 'stats.granDay': 'Jour', 'stats.granWeek': 'Semaine', 'stats.granMonth': 'Mois', 'stats.granYear': 'Année',
   'stats.confirmations': 'Confirmations', 'stats.paperQSL': 'QSL papier',
   
   'stats.byContinentSub': 'Part du journal',