feat(period): cut weeks and months on each game's reset clock

Check-in goals and weekly tasks both need a period longer than a day.
Working in game-day keys rather than instants means a week rolls exactly
where the game's own day does, per region, with no second clock.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/period.test.ts

@@ -0,0 +1,94 @@
+import { describe, expect, test } from "bun:test";
+import { dayKey, nextResetMs } from "../src/shared/daily.ts";
+import { addDays, isoWeekday, periodOf } from "../src/shared/period.ts";
+
+describe("isoWeekday", () => {
+  test("Monday is 1 and Sunday is 7", () => {
+    expect(isoWeekday("2026-10-12")).toBe(1);
+    expect(isoWeekday("2026-10-18")).toBe(7);
+  });
+});
+
+describe("addDays", () => {
+  test("crosses month and year ends", () => {
+    expect(addDays("2026-09-30", 1)).toBe("2026-10-01");
+    expect(addDays("2026-12-31", 1)).toBe("2027-01-01");
+    expect(addDays("2026-03-01", -1)).toBe("2026-02-28");
+  });
+});
+
+describe("periodOf: weeks", () => {
+  const week = (startsOn: number) => ({ unit: "week" as const, startsOn });
+
+  test("a Monday week holds Monday to Sunday", () => {
+    const p = periodOf(week(1), "2026-10-15");
+    expect(p.start).toBe("2026-10-12");
+    expect(p.days).toEqual([
+      "2026-10-12", "2026-10-13", "2026-10-14", "2026-10-15",
+      "2026-10-16", "2026-10-17", "2026-10-18",
+    ]);
+  });
+
+  test("the start day is its own week's first day", () => {
+    expect(periodOf(week(1), "2026-10-12").start).toBe("2026-10-12");
+    expect(periodOf(week(1), "2026-10-18").start).toBe("2026-10-12");
+  });
+
+  test("every start weekday lands on that weekday, on or before today", () => {
+    for (let s = 1; s <= 7; s++) {
+      const p = periodOf(week(s), "2026-10-15");
+      expect(isoWeekday(p.start)).toBe(s);
+      expect(p.start <= "2026-10-15").toBe(true);
+      expect(p.days).toHaveLength(7);
+      expect(p.days).toContain("2026-10-15");
+    }
+  });
+
+  test("a week can span a year end", () => {
+    expect(periodOf(week(1), "2027-01-01").days[0]).toBe("2026-12-28");
+  });
+
+  test("the week rolls at each region's Genshin reset, not at midnight", () => {
+    // Monday 2026-10-12's reset: 20:00Z Sunday in Asia, 03:00Z in Europe,
+    // 09:00Z in America.
+    for (const region of ["asia", "europe", "america"] as const) {
+      const reset = nextResetMs(Date.parse("2026-10-11T12:00:00Z"), region, "genshin");
+      const before = periodOf(week(1), dayKey(reset - 1, region, "genshin"));
+      const at = periodOf(week(1), dayKey(reset, region, "genshin"));
+      expect(before.start).toBe("2026-10-05");
+      expect(at.start).toBe("2026-10-12");
+    }
+  });
+
+  test("a game with a reset override rolls on its own hour", () => {
+    // hololive Dreams rolls at 05:00 JST (resetHourLocal), not the 04:00 default.
+    const reset = nextResetMs(Date.parse("2026-10-11T12:00:00Z"), "asia", "holodori");
+    expect(new Date(reset).toISOString()).toBe("2026-10-11T20:00:00.000Z");
+    expect(periodOf(week(1), dayKey(reset - 1, "asia", "holodori")).start).toBe("2026-10-05");
+    expect(periodOf(week(1), dayKey(reset, "asia", "holodori")).start).toBe("2026-10-12");
+  });
+});
+
+describe("periodOf: months", () => {
+  const month = (startsOn: number) => ({ unit: "month" as const, startsOn });
+
+  test("a month from the 1st", () => {
+    const p = periodOf(month(1), "2026-10-15");
+    expect(p.start).toBe("2026-10-01");
+    expect(p.days).toHaveLength(31);
+    expect(p.days.at(-1)).toBe("2026-10-31");
+  });
+
+  test("before this month's start day, the period began last month", () => {
+    const p = periodOf(month(16), "2026-10-15");
+    expect(p.start).toBe("2026-09-16");
+    expect(p.days.at(-1)).toBe("2026-10-15");
+  });
+
+  test("a start day past the month's end clamps to its last day", () => {
+    expect(periodOf(month(31), "2026-09-30").start).toBe("2026-09-30");
+    expect(periodOf(month(31), "2026-09-29").start).toBe("2026-08-31");
+    expect(periodOf(month(31), "2026-10-05").days[0]).toBe("2026-09-30");
+    expect(periodOf(month(30), "2027-02-28").start).toBe("2027-02-28");
+  });
+});