fix(period): [ and ] after month or year to date step by whole months or years

Month to date went back to 13-30 Sept because a partial range was moved
by its length; a range that starts a month, quarter, half or year now
steps by that unit.

src/lib/dates.ts

@@ -118,12 +118,23 @@ export function periodOf(value: string, view: ViewRange): Range {
   }
 }
 
+const UNITS: ViewRange[] = ["1M", "3M", "6M", "1Y"];
+
+function unitOf(range: Range, view: ViewRange): ViewRange | null {
+  const units = isDynamic(view) ? UNITS : [view, ...UNITS];
+  return (
+    units.find((unit) => {
+      const aligned = periodOf(range.start, unit);
+      return aligned.start === range.start && range.end <= aligned.end;
+    }) ?? null
+  );
+}
+
 export function shiftPeriod(range: Range, view: ViewRange, dir: 1 | -1): Range {
-  const fixed = isDynamic(view) ? "1M" : view;
-  const aligned = periodOf(range.start, fixed);
-  if (aligned.start === range.start && aligned.end === range.end) {
-    const anchor = dir === 1 ? addDays(range.end, 1) : addDays(range.start, -1);
-    return periodOf(anchor, fixed);
+  const unit = unitOf(range, view);
+  if (unit) {
+    const anchor = dir === 1 ? periodOf(range.start, unit).end : range.start;
+    return periodOf(addDays(anchor, dir), unit);
   }
   const days = daysBetween(range.start, range.end) + 1;
   return {