feat(frontend): race form, start time selects, calendar views, day page

- Hide org schedule fields when editing a past race; isRaceDateInPast helper
- StartTimeSelects (HH:mm:ss) and optional ?date= prefill on new race
- Full-card link to edit for races needing result entry; shadow token
- List/calendar toggle (sessionStorage); year grid and month focus views
- Date hover popover and /races/day/:ymd page with Add button
- Docs plan-korrektirovok-starty.md and startTime API note; client 0.4.0

Made-with: Cursor

frontend/src/lib/calendarUtils.ts

@@ -0,0 +1,49 @@
+import type { Race } from "../api";
+
+export const WEEKDAY_LABELS_SHORT_RU: string[] = ["Пн", "Вт", "Ср", "Чт", "Пт", "Сб", "Вс"];
+
+/** Monday-based week: Mon=0 … Sun=6 */
+export function mondayIndexFromDate(d: Date): number {
+  return (d.getDay() + 6) % 7;
+}
+
+/** Grid cells for one month: `null` = empty, `1..31` = day of month. Padded to full weeks, at least 6 rows. */
+export function buildMonthCells(year: number, monthIndex: number): (number | null)[] {
+  const first = new Date(year, monthIndex, 1);
+  const lead = mondayIndexFromDate(first);
+  const dim = new Date(year, monthIndex + 1, 0).getDate();
+  const cells: (number | null)[] = [];
+  for (let i = 0; i < lead; i += 1) {
+    cells.push(null);
+  }
+  for (let day = 1; day <= dim; day += 1) {
+    cells.push(day);
+  }
+  while (cells.length % 7 !== 0) {
+    cells.push(null);
+  }
+  while (cells.length < 42) {
+    cells.push(null);
+  }
+  return cells;
+}
+
+export function toYmd(year: number, monthIndex: number, day: number): string {
+  const m = String(monthIndex + 1).padStart(2, "0");
+  const d = String(day).padStart(2, "0");
+  return `${year}-${m}-${d}`;
+}
+
+export function groupRacesByYmd(races: Race[]): Map<string, Race[]> {
+  const map = new Map<string, Race[]>();
+  for (const race of races) {
+    const ymd = race.date.slice(0, 10);
+    if (!/^\d{4}-\d{2}-\d{2}$/.test(ymd)) {
+      continue;
+    }
+    const list = map.get(ymd) ?? [];
+    list.push(race);
+    map.set(ymd, list);
+  }
+  return map;
+}