docs(documents): explain monthBoundaryTo day-zero idiom (#385)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/document/timeline.ts

@@ -18,6 +18,9 @@ export function monthBoundaryFrom(yearMonth: string): string {
 
 export function monthBoundaryTo(yearMonth: string): string {
 	const [year, month] = yearMonth.split('-').map(Number);
+	// Day 0 of `month + 1` rolls back to the last day of `month` — so passing
+	// `month` (1-indexed) into `Date.UTC(year, month, 0)` lands on the last day
+	// of that month. Handles 28/29/30/31 and leap years without a lookup table.
 	const lastDay = new Date(Date.UTC(year, month, 0)).getUTCDate();
 	return `${yearMonth}-${String(lastDay).padStart(2, '0')}`;
 }