fix: sync sadhna calendar with local timezone

components/sadhna/streak-calendar.tsx

@@ -11,8 +11,8 @@
  *
  * Streak counters come from the dashboard payload; the grid itself is fed by
  * the /sadhna/history range endpoint, fetched per visible period and cached
- * so paging back to a period you've seen is instant. Everything is bucketed
+ * so paging back to a period you've seen is instant. "Today" is the browser's
- * by UTC calendar day to match the backend.
+ * local date, matching the backend's local-timezone day boundary.
  */
 
 import { useEffect, useMemo, useRef, useState } from "react";
@@ -26,19 +26,24 @@ const WEEKDAYS = ["Su", "Mo", "Tu", "We", "Th", "Fr", "Sa"];
 
 type View = "week" | "month";
 
-// ── UTC date helpers (operate on epoch-ms at UTC midnight) ──
+// Each calendar day is represented as the UTC-midnight epoch-ms of that date
+// (so the date string and all arithmetic stay tz-free and internally
+// consistent). "Today", however, is anchored to the browser's LOCAL date so
+// the day boundary matches the backend's local-timezone day reset.
 function ymd(ms: number): string {
   return new Date(ms).toISOString().slice(0, 10);
 }
-function startOfDayUTC(d: Date): number {
+// UTC-midnight ms whose calendar date is the browser's local today.
-  return Date.UTC(d.getUTCFullYear(), d.getUTCMonth(), d.getUTCDate());
+function localTodayMs(): number {
+  const d = new Date();
+  return Date.UTC(d.getFullYear(), d.getMonth(), d.getDate());
 }
 function startOfWeek(ms: number): number {
   return ms - new Date(ms).getUTCDay() * DAY;
 }
 
 export function StreakCalendar({ dash }: { dash: SadhnaDashboard | null }) {
-  const todayMs = startOfDayUTC(new Date());
+  const todayMs = localTodayMs();
   const todayStr = ymd(todayMs);
 
   const [view, setView] = useState<View>("week");