Add date range helpers for day tabs

src/features/online-board/components/DayTabs/dateRange.test.ts

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+import { describe, it, expect } from "vitest";
+import { formatYyyymmdd, parseYyyymmdd, generateDateRange, findPageIndex } from "./dateRange.js";
+
+describe("formatYyyymmdd", () => {
+  it("formats a Date to yyyymmdd", () => {
+    expect(formatYyyymmdd(new Date(2026, 3, 16))).toBe("20260416");
+  });
+
+  it("pads single-digit month and day", () => {
+    expect(formatYyyymmdd(new Date(2026, 0, 5))).toBe("20260105");
+  });
+});
+
+describe("parseYyyymmdd", () => {
+  it("parses yyyymmdd into Date", () => {
+    const d = parseYyyymmdd("20260416");
+    expect(d.getFullYear()).toBe(2026);
+    expect(d.getMonth()).toBe(3);
+    expect(d.getDate()).toBe(16);
+  });
+});
+
+describe("generateDateRange", () => {
+  it("generates dates from today-daysBefore to today+daysAfter inclusive", () => {
+    const today = new Date(2026, 3, 16);
+    const range = generateDateRange(today, 2, 3);
+    expect(range).toHaveLength(6);
+    expect(range[0]).toBe("20260414");
+    expect(range[2]).toBe("20260416");
+    expect(range[5]).toBe("20260419");
+  });
+
+  it("handles zero-range", () => {
+    const today = new Date(2026, 3, 16);
+    expect(generateDateRange(today, 0, 0)).toEqual(["20260416"]);
+  });
+
+  it("handles month boundaries", () => {
+    const today = new Date(2026, 3, 1);
+    const range = generateDateRange(today, 2, 0);
+    expect(range).toEqual(["20260330", "20260331", "20260401"]);
+  });
+});
+
+describe("findPageIndex", () => {
+  it("finds page index for the given date at pageSize=7", () => {
+    const dates = ["20260414", "20260415", "20260416", "20260417", "20260418", "20260419", "20260420", "20260421", "20260422"];
+    expect(findPageIndex(dates, "20260414", 7)).toBe(0);
+    expect(findPageIndex(dates, "20260420", 7)).toBe(0);
+    expect(findPageIndex(dates, "20260421", 7)).toBe(1);
+    expect(findPageIndex(dates, "20260422", 7)).toBe(1);
+  });
+
+  it("returns 0 when date not found", () => {
+    const dates = ["20260414", "20260415"];
+    expect(findPageIndex(dates, "20260999", 7)).toBe(0);
+  });
+});

src/features/online-board/components/DayTabs/dateRange.ts

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+/**
+ * Pure helper functions for generating and navigating date ranges
+ * in the DayTabs component. No React, no side effects.
+ */
+
+/** Format a Date as yyyymmdd (zero-padded). */
+export function formatYyyymmdd(d: Date): string {
+  const y = d.getFullYear().toString();
+  const m = (d.getMonth() + 1).toString().padStart(2, "0");
+  const day = d.getDate().toString().padStart(2, "0");
+  return `${y}${m}${day}`;
+}
+
+/** Parse a yyyymmdd string into a local Date. */
+export function parseYyyymmdd(s: string): Date {
+  const y = parseInt(s.slice(0, 4), 10);
+  const m = parseInt(s.slice(4, 6), 10) - 1;
+  const day = parseInt(s.slice(6, 8), 10);
+  return new Date(y, m, day);
+}
+
+/**
+ * Generate an inclusive list of yyyymmdd strings spanning
+ * [base - daysBefore, base + daysAfter].
+ */
+export function generateDateRange(base: Date, daysBefore: number, daysAfter: number): string[] {
+  const result: string[] = [];
+  for (let offset = -daysBefore; offset <= daysAfter; offset++) {
+    const d = new Date(base.getFullYear(), base.getMonth(), base.getDate() + offset);
+    result.push(formatYyyymmdd(d));
+  }
+  return result;
+}
+
+/**
+ * Find the page index (0-based) containing the given date, paginated by pageSize.
+ * Returns 0 if the date is not in the list.
+ */
+export function findPageIndex(dates: string[], target: string, pageSize: number): number {
+  const idx = dates.indexOf(target);
+  if (idx < 0) return 0;
+  return Math.floor(idx / pageSize);
+}