Cap chronotype window in the shared assembler

Make computeSleepRhythmFromNights own the chronotype look-back so the dashboard
summary (a year of nights) and GET /api/sleep/rhythm (six weeks) return the
same band, mid-sleep, and social-jetlag values rather than averaging different
samples. Derive the free/work day-type from the local wake-day digits so the
classification holds in far-east timezones, and read the shared sleep-rhythm
DTO type into the client hook instead of a hand-mirrored copy.

src/app/api/dashboard/summary/route.ts

@@ -661,7 +661,6 @@ async function buildDashboardSummary(
       sleepPriorityJson,
     ),
     sleepNeedMinutes(sleepProfile.ageYears),
-    userTz,
   );
 
   const metrics: MetricCard[] = [];